Experimental Study on the Effect of a Thermoresponsive Secundine

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Experimental Study on the Effect of a Thermoresponsive Secundine Inhibitor on Coal Spontaneous Combustion Chuanbo Cui,†,‡ Shuguang Jiang,*,†,‡,§ and Weiqing Zhang§ †

Key Laboratory of Coal Methane and Fire Control, Ministry of Education, ‡School of Safety Engineering, and §State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou, Jiangsu 221116, People’s Republic of China ABSTRACT: When a high-water-cut physical inhibitor (HWPI) is sprayed on the coal in goaf, moisture in coal will undergo a substantial reduction under the influence of liquid flow and water evaporation, which severely weakens the inhibitory effect of the HWPI and greatly shortens its inhibitory lifetime. Therefore, this paper proposes a thermoresponsive secundine inhibitor (TSI) to solve the problem of poor water retention and short inhibitory lifetime by sealing the HWPI in thermoresponsive secundine. Before the ambient temperature (30 °C) reaches the trigger temperature, the water retention rate of the TSI is 100%. After the trigger temperature is reached, the TSI will quickly release a large amount of the HWPI to suppress coal spontaneous combustion. The releasing time and trigger temperature of the TSI drop rapidly with the increase of the borehole diameter and borehole number, while its weight loss grows with the increase of the borehole diameter and borehole number. The experimental results reveal that the borehole diameter of 2.5 mm and the borehole number of 12 are the optimal conditions for the TSI to release more HWPI in a relatively short period of time.

1. INTRODUCTION Underground coal fire, a difficult and longstanding problem worldwide, is mainly associated with the spontaneous combustion of coal.1−4 It is well-known that coal can react with oxygen in air to cause an exothermic reaction, even in ambient conditions. Large coal area spontaneous combustion has occurred more frequently, and increased channels of air leak to coalmine goaf.5−7 A goaf refers to the area where the coal seam has been mined and a lot of coal is left. Because it is connected with the intake airway and return airway of the working face, the air leak in it is serious, making it a high incidence area of underground coal spontaneous combustion. Among China’s state-owned collieries, 56% of the mines have been jeopardized by coal spontaneous combustion, which leads to huge casualties, economic losses, and massive environmental contamination.8,9 Therefore, it is highly desirable to retard coal spontaneous combustion. Physical inhibitors are widely used to prevent or extinguish coal mine fires. At present, a variety of inorganic salts, such as NaCl, MgCl 2 , and CaCl 2 , have been considered10−12 as inhibitors for coal mine fires. By studying the controlling effects of 10 kinds of additives on bituminous coal spontaneous combustion, Smith et al.13 found that NaNO3, NaCl, and CaCO3 achieved the best inhibitory effect. Anthony et al.,14 Yucel et al.,15 Zhang et al.,16 and Li et al.17 studied the influence of moisture on coal spontaneous combustion. Liodakis et al.18 discovered that diammonium hydrogen phosphate and ammonium sulfate can effectively prevent coal from combusting spontaneously. Zhan et al.19 concluded that Na3PO4 can improve the thermal stability of coal and effectively inhibit the generation of free radicals. Taraba et al.20 and Vaclav et al.11 revealed that urea can effectively inhibit coal self-heating and spontaneous combustion. © XXXX American Chemical Society

It is well-known that coal keeps on oxidizing even at room temperature. The temperature of coal begins to rise when its heat release becomes greater than its heat loss,21,22 and coal oxidation will accelerate when the temperature rises to the critical temperature of spontaneous combustion.1,23 However, the temperature rise of residual coal in goaf from room temperature to the critical temperature is a very long process.6,7,9 The water in coal will evaporate in this process as a result of the air leak in goaf and the heat release of coal. The above-mentioned inorganic salt inhibitors are usually mixed with water to form aqueous solutions with a certain concentration. These inhibitors, however, retard coal oxidation merely by creating a barrier to oxygen and adsorbing water.24 As a result, water in the coal will evaporate in a large amount with the passage of time under the influence of the temperature. These compounds typically exhibit low efficiency and short active lifetimes.24 In this case, because of the weakening or even failure of inhibitory effects, these inhibitors fail to inhibit spontaneous combustion of coal in goaf, resulting in casualties and property losses. To solve the problem that the effect of physical inhibitor weakens or even fails as a result of water evaporation, this paper proposes a technique of encapsulating a high-water-cut physical inhibitor (HWPI) in thermoresponsive secundine (TS). The physical inhibitors are encapsulated in TS with boreholes, which are filled with paraffin. As we know, paraffin melts when heated. Therefore, when the ambient temperature reaches a certain value, the paraffin within TS laid above the coal in goaf will melt under the impact of heat, so that a thermoresponsive secundine inhibitor (TSI) releases the physical inhibitor to Received: September 19, 2017 Revised: November 9, 2017 Published: November 17, 2017 A

DOI: 10.1021/acs.energyfuels.7b02814 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

stainless-steel spherical shell. The stainless-steel spherical shell with boreholes was filled with HWPI, and the borehole was sealed with paraffin. The TS is a stainless-steel spherical shell drilled with a certain amount of holes, which were then filled with paraffin. The melting point, density, and specific heat capacity of paraffin were 53−58 °C, 0.9 g/cm3, and 2.14−2.9 J g−1 K−1, respectively. The key characteristic of paraffin, a component of the TSI, is that it turns from solid to liquid with the rise of the temperature. The spherical shell was 40 mm in diameter and 1.0 mm in thickness. The temperature at which the TSI starts to release the HWPI is defined as the trigger temperature of the TSI. 2.2. Experimental System. 2.2.1. Weighing System. The weighing system is a data acquisition system based on a temperature-programmed system, as shown in Figure 2. Consisting of a gas supply system, control panel, reaction vessel, heater, and thermocouple 1, the temperature-programmed system provides the required dry air and a certain rate of temperature rise.25 The data acquisition system contains a personal computer (PC), data acquisition center, electronic balance, and thermocouple 2. In the temperature-programmed process, the weighing system can measure the weight changes of the TSI in real time, to accurately obtain the trigger temperature of the TSI. Then, the experimental conditions of the weighing system are that the temperature was raised from 30 to 120 °C at a heating rate of 1 °C/min and a constant flow of 100 mL/min. 2.2.2. Testing System for Coal Acceleration Combustion Simulation. The testing system for coal acceleration combustion simulation is an improved version of the weighing system, as shown in Figures 2 and 3. The gas analysis system added for analyzing the indicator gas of coal spontaneous combustion comprises a highprecision CO sensor, CO2 sensor, O2 sensor, and signal processing module. Because this system can collect the concentration of indicator gas in real time, it is more efficient and convenient than a gas chromatograph. Besides, a condenser and bottle are added to collect the evaporated water; meanwhile, the electronic balance is used to measure the amount of water evaporation. The experimental conditions of the testing system are that the temperature was raised from 30 to 180 °C at a heating rate of 1 °C/min and a constant flow of 100 mL/min. 2.3. Experimental Methods. To efficiently inhibit coal spontaneous combustion, the residual rate and releasing time of the HWPI in the TSI should be minimized. Hence, this paper mainly studies the influence of the borehole diameter, number, and location on the TSI. 2.3.1. Experiment of the Influence of the Borehole Diameter on the TSI. First, experiments were carried out according to the following steps to study the influence of the borehole diameter on the trigger temperature, releasing time, and weight loss. Step 1: Two boreholes were drilled on the central axis of each of the six shells using 1.0, 1.5, 2, 2.5, 3.0, and 3.5 mm drill bits. Step 2: The six shells were injected with 30 g of HWPI using a syringe and sealed with paraffin. These spheres were the shaped TSI.

suppress low-temperature oxidation of coal. This temperature value is the trigger temperature for the TSI to release the inhibitor. Before the temperature is reached, the inhibitor within TS will not undergo water evaporation because it is protected by TS, and thus, its inhibitory effect will not be reduced. After the trigger temperature is reached, the water released from the HWPI will soak the coal. As a result, on the one hand, the temperature of coal will be lowered because heat is absorbed by water evaporation, while on the other hand, the contact between coal and oxygen will be prevented as water wraps the coal. In short, the TSI has the advantages of a long inhibitory life and a good inhibitory effect, and it can automatically release the inhibitor after reaching a certain temperature. It overcomes the defects of the existing inhibitors, which are incapable of controlling spontaneous combustion in coal seams or goaf as a result of their strong fluidity, short inhibitory function time, corrosivity, etc.

2. MATERIALS AND METHODS 2.1. Experimental Materials. The fresh coal used in the experiment was supplied by Longdong mine, Xuzhou, China. Through the proximate analysis, the contents of moisture, ash, volatile matter, and dry and free-ash volatile matter and true density of Longdong coal are 2.11, 16.52, 8.71, 10.35, and 1.99%, respectively. The coal was ground and sieved to a certain particle size (0.18−1.18 mm) and then dried under vacuum for 24 h at 40 °C. The HWPI adopted in this paper is a CaCl2 solution at the concentration of 20%. The TSI shown in Figure 1 consists of the HWPI, paraffin, and

Figure 1. Inhibitor mechanism of the TSI.

Figure 2. Weighing system based on the temperature-programmed system. B


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Figure 3. Testing system for coal acceleration combustion simulation. Step 3: Six TSIs with two boreholes were put into the reaction vessel in the weighing system, with the straight line crossing the two boreholes and TSI center being perpendicular to the ground. Meanwhile, the change of the TSI weight and temperature within the reaction vessel were collected by the weighing system. Step 4: The data were analyzed in accordance with the experimental results to obtain the optimal borehole value. 2.3.2. Experiment of the Influence of the Borehole Location and Number on the TSI. To obtain the influence of the borehole number and location on the TSI, this paper studied the conditions of 2, 4, 8, and 12 boreholes, respectively. These figures formed by the boreholes are internally connected to the TSI, as shown in Figure 4. The

First, 11 coal samples (200 g each) were prepared. One of the samples without the addition of reagent was taken as the raw coal sample. Another five samples, each added with four TSIs, were put into a 40 °C constant temperature vacuum drying oven and taken out to be weighed after 0, 6, 12, 18, and 24 h, respectively. Next, among the rest of samples whose initial moisture content was 33.75%, each sample was added with 120 g of HWPI. These samples were dried in a vacuum drying oven for 0, 6, 12, 18, and 24 h, respectively, and then taken out to obtain their weights and water contents. Lastly, the raw coal and coal with the HWPI dried for 12 and 24 h were put into the reaction vessel to perform the temperature-programmed experiment based on the testing system. 2.3.4. Experiment of the Inhibition of Low-Temperature Oxidation of Coal by the TSI. A total of 200 g of raw coal was evenly spread in the reaction vessel. Afterward, three TSIs were spread on the top of the coal. The borehole diameter and number of TSI are 2.5 mm and 2, respectively. Then, the experiment was performed on the basis of the testing system; meanwhile, the data were acquired. Finally, coal with two and one TSI were tested according to the above procedures.

3. RESULTS AND DISCUSSION As the ambient temperature of the TSI rises to a certain value, the paraffin on TS melts when its melting point is reached, allowing the inhibitor in TS to flow out through boreholes. The ambient temperature at which the inhibitor within TS starts to flow out is defined as the trigger temperature of the TSI; the time needed to release the inhibitor is defined as the releasing time; and the weight of the inhibitor flowing out from the TSI is defined as the weight loss of TSI. 3.1. Influence of the Borehole Diameter on the TSI. The TSI was placed in the programmed-temperature system where the ambient temperature rose at the rate of 1 °C/min. At the beginning, the weight of the TSI monitored by the weighing sensor remains unchanged. With the passage of time, when the ambient temperature reaches the trigger temperature, the weight of the TSI drops suddenly to a certain value and then remains stable, as shown in Figure 5. In Figure 5, the inflection point for the downward weight curve is the point for the TSI to release the inhibitor and the period from the beginning of the TSI weight drop to the stabilization of the TSI weight is the releasing time. It can be seen from Figure 6 that the inhibitor release rate grows significantly with the rise of the borehole diameter. Besides, within the borehole diameter of 1.0−2.5 mm, the releasing time decreases rapidly as the borehole diameter of the

Figure 4. Figures formed by different numbers of boreholes internally connected to the TSI: segment, regular tetrahedron, regular hexahedron, and regular icosahedron. Figure locations relative to the horizontal plane: (a) standing vertically and (b) lying lowly. influence of the location and number on the residual rate was studied according to the following steps. First, according to the results in section 2.3.1, the optimal borehole diameter was 2.5 mm. Second, eight stainless-steel spherical shells were selected and drilled with 2, 4, 8, and 12 boreholes. The borehole number, borehole location, and spherical shell placement were arranged in accordance with Figure 4. Lastly, the experiment was performed on the basis of the weighing system according to steps 2 and 3 in section 2.3.1. 2.3.3. Experiment of the Influence of Water Evaporation on Coal Spontaneous Combustion Inhibition by the HWPI. This paper comparatively studied the moisture-retaining capacity of the TSI and HWPI first and then researched the low-temperature oxidation ability of the HWPI-added coal sample after its water reduction. The experimental methods are as follows: C


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Figure 5. Weight of the TSI changing over time. Figure 7. Nonlinear fitting of the borehole diameter with the weight loss and trigger temperature.

diameter. The borehole diameter and trigger temperature satisfy the formula W = −27.31 exp(−r/0.58) + 29.65, with R2 = 0.83. To accurately measure the trigger temperature of the TSI, this experiment adopted a weighing sensor with a precision of 0.001 g and put the TSI on a screen hanging in the air. When the ambient temperature rises to a certain value, the paraffin on TS of different borehole diameters begins to melt simultaneously. However, affected by surface tension, the TSI with a smaller borehole diameter releases the inhibitor at a higher temperature. In short, to enable the TSI to achieve a better inhibitory effect, it is required to choose the borehole diameter corresponding to a shorter inhibitor-releasing time and larger weight loss. In light of the above results, the borehole diameter of 2.5 mm should be chosen. 3.2. Influence of the Borehole Number and Location on the TSI. As seen in Figures 8 and 9, the number and

Figure 6. Quadratic fit of the releasing time and borehole diameter.

TSI increases. When the borehole diameter exceeds 2.5 mm, the decreasing trend of the releasing time tends to be gentle. As shown in Figure 6, according to the measured data, the nonlinear fitting relationship between the borehole diameter and releasing time is t = 231.53r−2, where r is the borehole diameter, t is the releasing time of the inhibitor, and the fitting degree is 0.995 11. The inhibitory effect of the TSI is affected by not only the releasing time but also the weight loss and trigger temperature, because the weight loss and releasing time determine the inhibitor release rate of the TSI, while the trigger temperature determines the ambient temperature at which the TSI releases the inhibitor. Therefore, to achieve the best inhibitory effect, it is necessary to release more inhibitor within a shorter releasing time at a certain ambient temperature. As presented in Figure 7, with the increase of the borehole diameter, the weight loss increases rapidly, and the increasing trend of weight loss decelerates after the diameter reaches 2.5 mm. According to the measured data, the fitting relationship between the weight loss and borehole diameter is T = 84.17r−0.04, with R2 = 0.87. At the borehole diameter of 1.0 mm, the residual inhibitor in TS is 5.35 g. As the borehole diameter rises to 2.5 mm, the residual physical inhibitor in TS is basically 0. The reason for the above results is that the smaller the borehole diameter, the larger the influence of the surface tension on inhibitor solution at the borehole. When the inhibitor solution in TS is reduced to a certain value, the residual inhibitor solution cannot overcome the effect of the surface tension and, thereby, cannot be released. It can be seen from Figure 7 that the trigger temperature of the TSI decreases quickly with the increase of the borehole

Figure 8. TSI weight of different numbers of boreholes lying lowly

location of boreholes have a great influence on the TSI. When the boreholes lie lowly, the greater the borehole number, the larger the weight loss of the TSI. According to the comparative analysis between the theoretical calculation and experimental data, the minimum weight loss of the TSI in the same borehole number does not differ much. For the TSI drilled with 2 boreholes, the weight loss remains to be 0, even if the ambient temperature is much higher than the trigger temperature of the TSI. This is because, when the 2 boreholes are horizontally D


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Figure 10. Changes in moisture under different HWPI additions.

Figure 9. TSI weight of different numbers of boreholes standing vertically.

according to the experimental program in section 2.3. As known from Figure 10, the weight of raw coal with four TSIs remains almost constant within 24 h of constant temperature drying at 40 °C. Water does not evaporate because TSI will not release the inhibitor solution before the trigger temperature is reached. In contrast, the initial weight of the coal sample directly sprayed with the inhibitor is 320 g. After 6 h, its weight is reduced by 88.91 g, and its moisture decreases at the rate of 14.82 g/h; meanwhile, its moisture content drops from 33.75 to 13.45%, because the fluidity of the HWPI causes the loss of most of the HWPI when it is directly sprayed on residual coal. Thus, the main cause of this result is the loss of moisture, and the secondary reason is the evaporation of water. When the drying time increases from 6 to 24 h, the water content decreases from 13.45 to 3.22%, because the moisture reduction rate of the sample decreases gradually and the water evaporation rate is 1.36 g/h. In this process, water evaporation is the main reason for the reduction of the water content. Figure 11 shows that, as the temperature rises, the weight of water evaporated from the reaction vessel gradually increases.

located, water around boreholes is not able to flow out because it is impacted by the greater surface tension than hydraulic pressure. For the TSI drilled with 4 boreholes, one of the holes is at the top of the spherical shell, while the other holes are distributed horizontally in the middle and lower parts of the spherical shell. When the ambient temperature reaches the trigger temperature of the TSI, water around the boreholes in the middle and lower parts flows out because it is impacted by the smaller surface tension than hydraulic pressure. To the same token, for the TSI drilled with 8 and 12 boreholes, the TSI will release the HWPI when the ambient temperature reaches the trigger temperature, as listed in Table 1. Because the boreholes are distributed according to Figure 4a, the weight loss of the TSI grows with the increase of the borehole number. Table 1. Influence of the Borehole Number and Location on the TSI number of boreholes minimum weight loss by theory calculation (%) minimum weight loss by experiment (%) maximum weight loss (%) trigger temperature (°C) minimum releasing time (s) maximum releasing time (s)

4

8

12

0.00

2

74.07

88.49

97.39

0.00

62.45

87.48

95.83

100 81.9 26 no release

100 81.1 296 415

100 80.3 165 315

100 79.8 73 182

When the boreholes stand vertically, there are always two boreholes located at the top and bottom of the spherical shell. Therefore, when the ambient temperature reaches the trigger temperature, the HWPI in the TSI flows out and the weight is lost for 100%. The only difference lies in the releasing time as a result of different borehole numbers. The releasing time is the shortest for the TSI with 2 boreholes and the longest for the TSI with 4 boreholes. Besides, the releasing time is rapidly shortened when the number of boreholes increases from 4 to 12. To allow the TSI to release more HWPI in a short period of time, it is best to select the TSI with 12 boreholes based on the results listed in Table 1. 3.3. Influence of Moisture Reduction on the Coal Spontaneous Combustion Inhibition Effect of the HWPI. The experimental results shown in Figure 10 can be obtained

Figure 11. Weight of water evaporation of different coal samples.

The larger the initial moisture content of the sample, the greater the water evaporation rate of the sample during the temperature rise. It can be seen from Figure 11 that water evaporation of raw coal with HWPI dried for 0 h starts to increase at 65 °C, that of raw coal with HWPI dried for 12 h starts to increase at 75 °C, and that of raw coal with HWPI dried for 24 h starts to increase at 80 °C. Raw coal with HWPI E


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Figure 13. Weight of water evaporation among different coal samples.

Figure 12. CO concentration of different coal samples. Figure 14. Water evaporation rate of different coal samples.

Moreover, at the same temperature, among the samples dried for 0, 12, and 24h, the sample with a lower initial moisture content can produce more CO. This is because the coal sample with more moisture will absorb more heat during the heating process, thereby inhibiting the oxidation of coal. Besides, water will form a liquid film on the surface of the sample, which will insulate the sample from oxygen. Thus, the higher the initial moisture content, the weaker the low-temperature oxidation ability of the coal sample. Even if HWPI is added to the coal sample, water loss and evaporation will still greatly weaken the effect of the inhibitor on the suppression of low-temperature oxidation of coal. As a result, it is very important to study a new type of inhibitor, which has good water retention, good inhibitory effect, and a long lifetime. 3.4. Comparative Analysis of the Inhibitory Effects and Lifetimes of the TSI and HWPI. During the programmed-temperature process, the amount of water evaporation in the temperature range of 30−70 °C is very small and negligible, increases slowly in the temperature range of 70−105 °C, and increases at an obviously higher rate from 105 °C, as shown in Figure 13. At the same temperature, the sample with a higher water content exhibits a higher evaporation rate, as shown in Figure 14. Water of the sample with the TSI rarely evaporates before the temperature of the reaction furnace reaches 80 °C, whereas the sample directly sprayed with HWPI begins to evaporate at 70 °C. In the range of 70−105 °C as a result of good thermal conductivity of the sample directly sprayed with HWPI, its water evaporates faster than the sample with TSI. At 80 °C, thermal conductivity of the sample with TSI is enhanced after the TSI releases the inhibitor

solution. In comparison to the sample directly sprayed with HWPI, the sample with TSI starts to experience an increase in its water evaporation at a higher temperature but its high moisture content determines that its water evaporates at a higher rate after 100 °C. With the rise of the temperature, the amount of CO produced by each sample increases gradually, as does the amount of water evaporation. Before 110 °C, in comparison to the raw coal, the sample directly sprayed with HWPI solution has a certain inhibitory effect, while the sample with TSI exerts no inhibitory effect and even promotes coal oxidation. This is because the coal sample directly sprayed with HWPI contains water, which can envelop the sample and insulate it from oxygen. Meanwhile, during the temperature rising process, water evaporation will absorb large amounts of heat, thus lowering the temperature of coal. At the same temperature, among the samples directly sprayed with HWPI, the sample with a lower initial moisture content exerts a weaker inhibitory effect. In addition, with the rise of the temperature, the sample with a lower initial moisture content shows a more notable trend of weakening inhibitory effect. Although the TSI does not exhibit a desirable inhibitory effect before 110 °C, its inhibitory effect is significantly better than that of the sample directly sprayed with HWPI after 110 °C. As mentioned previously, the TSI does not release the HWPI before reaching the trigger temperature (about 80 °C); therefore, the sample with TSI produces the same amount of CO as the raw coal sample in this period. When the trigger temperature is reached, the sample F


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temperature after 160.6 °C. This is mainly because the oxidation of raw coal starts to accelerate from 120 °C, and the accumulation of heat released by the oxidation allows the raw coal temperature to exceed the programmed temperature after 160.6 °C. The temperatures of raw coal and raw coal with HWPI dried for 24 h cross with the programmed temperature at 160.6 and 169.6 °C, respectively, as indicated by arrows in Figure 16. For the rest of coal samples, their temperatures are all lower than the programmed temperature in the range of 30− 200 °C. The temperatures of samples added with TSI rise at a higher rate in the range of 80−120 °C, because the HWPI released by TSI at 80 °C would wet and coat the coal to enhance their thermal conductivity. In addition, in coal, there is a huge internal surface area. When the coal is moistened by moisture, the water interacts with the surface of the coal and releases a certain amount of heat of wetting to promote the coal oxidation. The heat of wetting decreases with the increase of the moisture content in coal. After 120 °C, the sample added with more TSI experiences a larger amount of heat absorption by water evaporation, and thereby its temperature rises at a lower rate. Before the programmed temperature experiment, water contents of raw coal with HWPI dried for 0 h and raw coal with HWPI dried for 12 h are 33.75 and 9.5%, respectively; thus, their temperatures rise relatively faster in the range of 70− 110 °C. Besides, because the former has a larger water content than the latter, its heating rate is lower than that of the latter after 130 °C. Finally, it can be seen from Figure 16 that the heating rates of samples added with TSI are lower than those of samples directly sprayed with HWPI.

with TSI produce more CO because TSI releases large amounts of HWPI and then water soaks the coal, increasing its thermal conductivity. After 110 °C, a large amount of water released by TSI evaporates rapidly and absorbs a great deal of heat, so that the coal temperature is lowered and the coal is insulated from oxygen. It can be found from Figure 15 that, for the samples,

Figure 15. CO concentration of different coal samples.

the higher the moisture content, the more obvious the inhibitory effect. Moreover, the inhibitory ability of three TSIs is close to that of 120 g of HWPI. For the samples directly sprayed with HWPI, the longer the drying time, the lower the moisture content and, thus, the weaker the inhibitory effect. Therefore, to enable the inhibitor to possess a better inhibitory effect and a longer inhibitory lifetime, an inhibitory technique that can not only stop water evaporation but also release large amounts of HWPI in time is needed. The TSI proposed in this paper can just solve this problem. Figure 16 shows changes of the programmed temperature and the temperatures of samples in the reaction vessel over

4. CONCLUSION A novel approach involves the use of TSI to solve the problem that the effect of the physical inhibitor is weakened and the inhibitory life is shortened as a result of water evaporation. The TSI has a good water retention rate of 100% before the ambient temperature reaches the trigger temperature. When the ambient temperature reaches the trigger temperature, the TSI releases the HWPI, so that water wets and envelops the coal sample, thus insulating it from oxygen. Meanwhile, the water evaporates and absorbs heat, thus preventing the temperature of the sample from rising. In addition, the TSI boasts both good water retention and a long inhibitory lifetime. The programmedtemperature experiments indicate that the TSI also has a good inhibitory effect. In this paper, the effects of the borehole diameter, number, and location on the weight loss, release time, and trigger temperature of the TSI were investigated through experiments. The results reveal that the TSI can achieve the best inhibitory effect when the borehole diameter, number, and location are 2.5 mm, 12, and regular icosahedron, respectively. At the same time, the releasing time, trigger temperature, and weight loss of the best inhibitory effect are 73−182 s, 79.8 °C, and 95.83−100%, respectively.

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Corresponding Author

Figure 16. Temperature−time curves under different conditions.

*Telephone: 0086-516-83885159. E-mail: [email protected]. cn.

time. As seen from Figure 16, the programmed temperature shares a linear relationship with time; that is, the temperature rises by 1 °C every 1 min. The temperature of raw coal in the reaction vessel is lower than the programmed temperature before 160.6 °C, while it is higher than the programmed

ORCID

Chuanbo Cui: 0000-0002-8765-5274 Notes

The authors declare no competing financial interest. G


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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (2017CXNL02) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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