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5БЕЗОПАСНОСТЬ ДЕЯТЕЛЬНОСТИ ЧЕЛОВЕКА HUMAN LIFE SAFETY
Original article / Оригинальная статья УДК 622.82:552
DOI: https://doi.org/10.21285/2500-1582-2019-4-430-439
Experimental study on the effect of acidity on coal spontaneous combustion at differ-ent oxygen concentrations
Xueming Li, Gang Bai
College of Safety Science and Engineering, Liaoning Technical University, Fuxin Liaoning, China
Key Laboratory of Mine Thermodynamic Disasters and Control of Ministry of Education, Fuxin Liaoning, China
Abstract: In order to study the effect of acidity and different oxygen concentration on coal spontaneous combustion under carbon dioxide-oxygen conditions, Changcun coal was investigated by the thermal gravimetric test .The combustion characteristics of neutral coal and hydrochloric acid sample were discussed under oxygen concentration of 20%, 15%, 11.25%, 7.5%, 3.75% and 2.5% respectively. The results showed that the TG-DSC curves shift back to the high temperature region, the characteristic temperature point increased correspondingly, and the heat release rate becomes smaller with the decrease of oxygen concentration. As the temperature increased, the heat release increase rapidly, and the difference of heat value between the different oxygen concentrations gradually became larger. At 800K, the difference of heat values from neutral coal between the maximum and the minimum oxygen concentration was biggest, approximately 11701J/g and that of acidic coal was about 11510J/g. At the same oxygen concentration, the heat value of the neutral coal was greater than that of the acidic coal sample, but it was not obvious when the oxygen concentration was lower than 7.5%. And according to the heat release, critical oxygen concentration of Changcun lean coal is 3.75%.
Key words: coal spontaneous combustion, different oxygen concentrations, acidic coal samples, heat release, characteristic temperature point
Information about the article: Received September 3,, 2019; accepted for publication October 3, 2019; available online December 31, 2019.
For citation: Li Xueming, Bai Gang. Experimental study on the effect of acidity on coal spontaneous combustion at different oxygen concentrations. XXI century. Technosphere Safety. 2019;4(4):430-439. https://doi.org/10.21285/2500-1582-2019-4-430-439
Экспериментальное исследование влияния кислотности на самопроизвольное возгорание угля при разных концентрациях кислорода
Сюэмин Ли, Ганг Бай
Колледж инженерии и вопросов безопасности, Ляонинский технический университет, Фусинь Ляонин, Китай Лаборатория горных термодинамических аварий и контроля Министерства образования, Фусинь Ляонин, Китай
Аннотация: С целью изучения влияния кислотности и концентрации кислорода на самовозгорание угля уголь месторождения Чанцунь был исследован с использованием термогравиметрического метода. Характеристики возгорания нейтрального угля и соляной кислоты исследуются при концентрации кислорода 20%, 15%, 11,25%, 7,5%, 3,75% и 2,5%. Результаты показали, что кривые TG-DSC смещаются в сторону, противоположную области высоких температур. Температура увеличивается, а скорость тепловыделения уменьшается с уменьшением концентрации кислорода. По мере увеличения температуры тепловыделение увеличивается, и разница в тепловой способности при различных концентрациях кислорода увеличивается. При 800 К разница тепловых показателей между максимальной и минимальной концентрацией кислорода была больше у нейтрального угля - 11701 Дж / г; для кислого угля она составила 11510 Дж / г. При той же концентрации кислорода значение тепловых показателей нейтрального угля было больше, чем у кислого угля. Это не было очевидно при концентрации кислорода ниже 7,5%. Критическая концентрация кислорода в угле составила 3,75%.
Ключевые слова: самовозгорание угля, концентрации кислорода, образцы кислого угля, тепловыделение, точка температуры
Xueming Li, Gang Bai. Experimental study on the effect of acidity on coal spontaneous combustion
at differ-ent oxygen concentrations Сюэмин Ли, Ганг Бай. Экспериментальное исследование влияния кислотности на самопроизвольное возгорание угля при разных концентрациях кислорода
Информация о статье: Дата поступления 3 сентября 2019 г.; дата принятия к печати 3 октября 2019 г.; дата онлайн-размещения 31 декабря 2019 г.
Для цитирования: Ли Сюэмин, Бай Ганг. Экспериментальное исследование влияния кислотности на самопроизвольное возгорание угля при разных концентрациях кислорода. XXI век. Техносферная безопасность. 2019;4(4):430—439. https://doi.org/10.21285/2500-1582-2019-4-430-439
1. Introduction
Mine fires are common around the world and are one of the natural disasters in coal mines. Fifty-six percent of China's mines have problems with spontaneous combustion. Mine fire will cause casualties, environmental pollution and loss of coal resources [1-4]. Injection of inert gas [5] and spraying of chemical retardants can inhibit spontaneous combustion. There are many kinds of retardants [6-8]. Because of the low price and good retarding effect of chloride salt retardants, most mines use them to prevent coal spontaneous combustion. Pandey [9] studied the influence of different retardants on spontaneous combustion characteristics and selected the best fire retardants for the site. Mohalik et al. [10] use inert gas to control underground fires. Shao et al. [11] study the effects of carbon dioxide and nitrogen on coal oxygen consumption and CO generation rate by means of coal spontaneous combustion heating program, and prove that carbon dioxide inhibition effect is stronger than nitrogen by means of activation energy. Wang et al. [12] apply nitrogen and carbon dioxide inerting into the coal in advance for 12h, and then use oil bath heating experiment system to compare and analyze the parameter change characteristics, CO generation rate and oxygen consumption rate during the programmed heating experiment, and also obtain that the carbon dioxide inhibition effect is good. Huang et al. [13] study the effects of oxygen concentration, particle size and heating rate on the combustion characteristics of coal in O2/CO2 atmosphere. The results show that oxygen concentration plays the most important role. Wen et al.[14] study the effect of variable oxygen concentration on the spontaneous combustion characteristics of
coal in the temperature programmed experiment. It is considered that the effect of oxygen concentration on spontaneous combustion is very complex. Zhang et al [15] think that there will be a larger exothermic peak with the increase of oxygen concentration, but the influence on ignition temperature of coal spontaneous combustion can be neglected. Some scholars use spraying retardants and inert gas to strengthen fire control [16].
Scholars have investigated the effects of different oxygen concentrations, inert gases and retardants on the spontaneous combustion characteristics of coal. However, it is not taken into account that chloride salt retardants such as magnesium chloride (MgCl2) and calcium chloride (CaCl2) will be weak acidity when dissolved in water, and whether coal soaked in weak acidity will affect the injection of carbon dioxide. In addition, due to air leakage, various gases released by blasting and spontaneous combustion of coal and other factors, the oxygen concentration in mine is not constant. In this paper, the lean coal in Changcun coal mine is taken as the research object, and the combustion characteristics of neutral coal and acidic coal in carbon dioxide-oxygen atmosphere under variable oxygen concentration were studied by thermogravi-metric analyzer. The experimental results can be used to determine the reasonable injection amount of carbon dioxide in Changcun coal mine and to explore whether the weak acidity of the retardants dissolved in water has an effect on the injection of carbon dioxide.
2. Experimental
Sample selection and preparation. In this paper, the coal samples from Changcun Coal Mine were selected as the experimental
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БЕЗОПАСНОСТЬ ДЕЯТЕЛЬНОСТИ ЧЕЛОВЕКА HUMAN LIFE SAFETY
coal samples and sealed immediately after sampling and transported back to the laboratory. Strip off the outer layer of lump coal and take fresh coal from the coal core. Then the lump coal was crushed by a crusher, and the coal samples with particle size of 100-250 mesh were selected by screening. The experimental coal samples were divided into two kinds, one was neutral coal and the other was acidic coal. The acidic coal sample was made by adding hydrochloric acid solution of pH=5 to the raw coal at a ratio of 1mL: 1g. After stirring uniformly, the coal sample was immersed for one week to react fully with it. The production of neutral coal sample only changes hydrochloric acid into distilled water, other operations remain unchanged. The neutral and acid-
3. Results and discussion
Mass loss analysis of coal combustion. According to the TG curves, the mass loss of coal sample can be obtained, as shown in Table 2. From Fig. 1, Fig. 2 and Table 2, the TG curve trend of each coal sample is basically the same. With the decrease of oxygen concentration, the TG curve is shifted to higher temperatures, and the total time of combustion of coal is prolonged, and mass loss is not significant. Due to the decrease of oxygen concentration, Coal adsorption of oxygen is reduced, mass gain is decreased. The reason for this phenomenon is that when the oxygen concentration is lowered, the coal sample is in an oxygen-poor state, and the active material in the coal cannot sufficiently react with oxygen, and the coal-oxygen compounding is difficult, and the combustion is inhibited. Espe-
ic coal samples were dried in a vacuum drying chamber at 60°C for 12 hours, and then removed to a wide-mouth bottle for reserve. The proximate analysis is shown in Table 1.
Experimental procedure and apparatus. Tests were carried out using a Netzsch STA 449 F5 thermal analyzer with constant heating rate of 5 Kmin-1 in the range 301K-1073K and all the tests were performed with a sample size of 15 mg. The total flow rate of the protection and carrier gases is kept constant at 80 mLmin-1. The experimental gas was the mixture of oxygen and carbon dioxide with various mixing ratios. The oxygen concentration was 2.5%, 3.75%, 7.5%, 11.25%, 15% and 20%, respectively.
Table 1 Таблица 1
cially when the oxygen concentration is 3.75% and 2.5%, the inhibitory effect is more pronounced than other oxygen concentrations. The effect of hydrochloric acid on mass is not obvious.
Characteristic temperature point analysis. There is a turning point in the combustion process of coal, and the corresponding temperature of these turning points is called the characteristic temperature point. The characteristic temperature point can reflect the influence of external conditions on the development process of coal spontaneous combustion at a macro level and provides a concrete basis for studying spontaneous combustion oxidation of coal [17]. The characteristic temperature points (shown in Fig. 3 and Table 3) of the whole oxidation process can be determined by TG-DSC curve.
Proximate analysis of experimental coal samples
Предварительный анализ экспериментальных образцов угля
Sample Mad, % Aad, % Vdaf, % FCad, %
Changcun coal 1.25 9.17 13.24 76.34
Xueming Li, Gang Bai. Experimental study on the effect of acidity on coal spontaneous combustion
at differ-ent oxygen concentrations Сюэмин Ли, Ганг Бай. Экспериментальное исследование влияния кислотности на самопроизвольное возгорание угля при разных концентрациях кислорода
110 г
100 —
90 -
80 - -20% -15%
70 - -11.25%
60 0х - -7.5% 3.75%
О 50 н - -2.5%
40 -
30 -
20 -
10 -
0
300 400 500
600 700 800 Temperature/K
4 2 0
_I_I_
900 1000 1100
10Jg
_ о
6 И
Q
Fig. 1. TG - DSC curves of neutral coal under different oxygen concentrations Рис. 1. TG-DSC кривые нейтрального угля при различных концентрациях кислорода
110 100 90 80 70 -я 60
ох
О 50
н
40 30 20 10
0
1 18 - 16
- 14
- 12
-8 Si -Ǥ
" и -4 Q
- 2
- 0
300 400 500 600 700 800 900 1000 1 Temperature/K
-J -2 100
Fig. 2. TG-DSC curves of acidic coal under different oxygen concentrations Рис. 2. TG-DSC кривые кислого угля при различных концентрациях кислорода
Weight loss and weight gain of two kinds of coal Потеря веса и увеличение веса двух видов угля
Table 2 Таблица 2
Oxygen concentration,% Mass loss of neutral coal, % Mass gain of neutral coal, % Mass loss of acidic coal, % Mass gain of acidic coal, %
20 89.85 3.22 90.35 3.19
15 88.39 3.44 86.41 3.77
11.25 88.43 3.33 87.97 3.32
7.50 87.78 2.91 86.75 3.33
3.75 88.52 3.22 86.88 2.46
2.50 88.51 1.79 88.62 1.95
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The characteristic temperature of coal samples Температура образц
Oxygen concentration, % Initial exothermic temperature of neutral coal, K Initial exothermic temperature of acidic coal, K Ignition temperature of neutral coal, K Ignition temperature of acidic coal, K Maximum heat flow rate temperature of neutral coal, K Maximum heat flow rate temperature of acidic coal, K
20 333.65 344.15 720.75 722.85 762.25 756.95
15 344.45 350.15 736.45 737.25 775.55 774.65
11.25 343.95 353.45 740.65 746.55 789.75 818.95
7.5 336.45 345.65 755.35 759.55 809.95 810.05
3.75 349.05 352.85 777.95 780.85 847.65 853.55
2.5 343 355 786 789 867 864
ов угля
Table 3
Таблица 3
870
840
810
iI 780
Sa S ü
н 750
720 360: 330
Initial exothermic temperature of neutral coal Initial exothermic temperature of acidic coal Ignition temperature of neutral coal Ignition temperature of acidic coal Maximum heat flow rate temperature of neutral coal Maximum heat flow rate temperature of acidic coal Maximum heat flow rate temperature of acidic coal
6 8 10 12 14 16 Oxygen concentration/%
18 20 22
2
4
Fig. 3. The change rule of coal sample temperature point under different oxygen concentrations
Рис. 3. Правило изменения температурной точки образца угля при различных
концентрациях кислорода
Fig. 3 shows that with the decrease of oxygen concentration, the initial exothermic temper-ature [18], ignition temperature and the maximum heat flow rate temperature increase. Compared with the oxygen concentration of
20%, when the oxygen concentration is 2.5%, the initial exothermic temperature of neutral coal sample delays by 10K, ignition temperature increases by 65.7K and the maximum heat flow rate temperature rises by 105.4K;
Xueming Li, Gang Bai. Experimental study on the effect of acidity on coal spontaneous combustion
at differ-ent oxygen concentrations Сюэмин Ли, Ганг Бай. Экспериментальное исследование влияния кислотности на самопроизвольное возгорание угля при разных концентрациях кислорода
the initial exothermic temperature of acidic coal sample delays by 10K, ignition temperature increases by 65.9K and the maximum heat flow rate temperature rises by 107.1K. Increasing the ignition temperature can prolong the temperature before ignition, reduce the oxidation time of coal in oxidation zone and reduce the spontaneous combustion probability. Comparing the characteristic temperature of acidic coal and neutral coal at the same oxygen concentration, it can be found that the initial exothermic temperature, ignition temperature and the maximum heat flow rate temperature of acidic coal sample are slightly higher than that of neutral coal. The maximum difference of initial exothermic temperature, ignition temperature and the maximum heat flow rate temperature between neutral coal and acidic coal is 12K, 6K and 9K respectively. This indicates that the addition of hydrochloric acid reduces the chemical oxidation of coal sample [19], and has a restraining effect on the spontaneous combustion process of coal.
Heat release analysis. According to coal-oxygen compounding theory, the root cause of coal spontaneous combustion is the release of heat from oxidation, which is the exothermic nature of coal. In a good heat stor-
age environment, when the amount of heat produced by oxidation is greater than the loss of heat to the environment, the coal will keep heat up until leading to spontaneous combustion [19-20]. At the beginning of the thermo-gravimetric experiment, small molecule gases and evaporating water were released from coal to absorb heat. Then, as the experiment was carried out, the coal undergoes physical absorption, chemical adsorption, and the chemical reaction to release heat. As shown the DSC curves in Fig. 1 and 2, as the oxygen concentration decrease, the curve gradually changes from a sharp peak to a flat peak, and the extreme value of the heat flow rate changes accordingly. The neutral coal sample reduces from 16.25 mW/mg to 7.9 mW/mg, with the changes of 8.35 mW/mg. The acidic coal sample reduces from 16.32 mW/mg to 7.472 mW/mg, with the changes of 8.848 mW/mg. The extreme value of the heat flow rate of the acid coal sample is greater than that of the neutral coal, and the acid coal sample is also higher than the neutral coal at the extreme point temperature. According to the DSC curve data, using the Origin software is used to calculate the heat release of the two coal samples at different oxygen concentrations, and drawings were made as Fig. 4 and 5.
400 500 600 700 800 900 1000 1100 Temperature/K
600 700 800 Temperature/K
Fig. 4. At different oxygen concentrations, heat release trend with temperature Рис. 4. Тенденция тепловыделения при разных концентрациях кислорода
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25000
20000
20000
5000
5000
0000
0000
5000
5000
0
0
400
500
900
1000
Temperature/K Temperature/K
400 500 600 700 800 900 1000 Temperature/K
8 12000
500 600 700
Temperature/K
0
400
800
900
Fig. 5. Comparison of heat release of neutral coal and acidic coal samples Рис. 5. Сравнение тепловыделения для образцов нейтрального и кислого угля
From Fig. 4, it can be seen that when coal and oxygen undergo physical adsorption, chemical adsorption and chemical reaction, the heat release gradually increases. But at the beginning, the rate of heat release is slow and the heat release is small. The heat increase can not be seen clearly through the curve. After about 500K, the heat release gradually increases, and the difference of heat release between different oxygen concentra-
tions gradually appeared. At 800K, the difference of heat release between the highest and lowest oxygen concentration is the largest, the difference of heat release from neutral coal is about 11701J/g, and the difference of heat release from acidic coal is about 11510J/g. According to Fig. 5, at the same oxygen concentration, the heat release of neutral coal and acidic coal samples is different. With the increase of oxygen concentration, the heat re-
Xueming Li, Gang Bai. Experimental study on the effect of acidity on coal spontaneous combustion
at differ-ent oxygen concentrations Сюэмин Ли, Ганг Бай. Экспериментальное исследование влияния кислотности на самопроизвольное возгорание угля при разных концентрациях кислорода
lease of acidic coal samples is less than that of neutral coal. However, when the oxygen concentration is 2.5%, the difference of heat release between neutral and acidic coal is little, which indicates that the oxygen inhibition effect is more obvious at low oxygen concentration, and acidity has little influence on it. It can also be found from DSC data that the initial exothermic temperature of acidic coal samples at various oxygen concentrations is also delayed compared with that of neutral coal, so the endothermic stage of acidic coal samples is prolonged. The addition of hydrochloric acid prolongs the initial exothermic temperature, increases the endothermic stage, decreases the release heat, reduces the thermal storage of coal body itself, and inhibits rapid heating of coal.
Determine the critical value of oxygen density. According to Fig. 4, the influence of oxygen concentration on heat release rate gradually increased after 500K. As the oxygen concentration decreases, the slope of the growth of the heat release curve decreases, and the growth trend of heat release is inhibited. In addition, four heat lines whose oxygen concentration is between 20% and 7.5% are close to each other, 3.75% and 2.5% are close to each other. While 7.5% and 3.75% are far away, and there is a significant difference. Oxygen 3.75% is the obvious dividing concentration of coal exothermic, so when underground oxygen con-centration reduce to at least 3.75%, the spontaneous combustion can be obviously suppressed. The experimental results show that the exothermic rate of coal sample is no longer attenuated after the oxygen concentration is lower than 3.75%. It can be judged by the heat release curve that when the oxygen concentration is reduced to 3.75%, it is close to the optimum inhibitory concentration. If oxygen concentration continue to reduce, it not only increase the cost of the mine, but also have no great inhibitory effect. Hence, it can also determine the minimum amount of carbon dioxide injected into the mine and pro-
vide the most reasonable suggestions for mine gas injection.
4. Conclusions
1. According to the TG-DSC curves, with the decrease of oxygen concentration, the whole combustion process will lag behind. The law of mass change is not obvious when hydrochloric acid is added.
2. As oxygen concentration decreases, each characteristic temperature increases correspondingly. The initial exothermic temperature of neutral coal changes the least, increasing by 10K, ignition temperature increases by 65.7K and the maximum heat flow rate temperature rises by 105.4K; the initial exothermic temperature, ignition temperature and the maximum heat flow rate temperature of acidic coal sample delays by 10K, 65.9K and 107.1K. The characteristic temperature of acid coal samples is higher than that of neutral coal. The maximum difference of initial exothermic temperature, ignition temperature and the maximum heat flow rate temperature between neutral coal and acidic coal is 12K, 6K and 9K respectively.
3. With the increase of temperature, the heat release increases rapidly, and the difference of heat between different oxygen concentration become apparent. At 800K, the difference of heat release between the highest and lowest oxygen concentration is the largest, the difference of heat release from neutral coal is about 11701J/g, and that of acidic coal is about 11510J/g. The heat release trend of acidic coal sample is the same as that of neutral coal, and the heat release of acidic coal sample is smaller at the same oxygen concentration. Therefore, the addition of hydrochloric acid has a certain inhibition effect on coal exothermic.
4. According to the heat release curves, the heat release of 3.75% and 2.5% oxygen concentration differs greatly from that of 7.5% oxygen concentration. That is, when the oxygen con-centration drops to 3.75%, the
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inhibiting effect is not obvious. It is concluded that the critical oxygen concentration is 3.75%. This result can also provide the most reason-
able suggestion for determining the minimum amount of carbon dioxide injected into the mine.
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Xueming Li, Gang Bai. Experimental study on the effect of acidity on coal spontaneous combustion
at differ-ent oxygen concentrations Сюэмин Ли, Ганг Бай. Экспериментальное исследование влияния кислотности на самопроизвольное возгорание угля при разных концентрациях кислорода
Contribution
Xueming Li, Gang Bai have equal authors' rights and responsibility for plagiarism.
Критерии авторства
Сюэмин Ли, Ганг Бай имеют равные авторские права и несут равную ответственность за плагиат.
Conflict of interests
The authors declare no conflict of interests.
The final manuscript has been read and approved by all the co-authors.
Information about the authors Xueming Li,
College of Safety Science & Engineering,
Liaoning Technical University,
Fuxin Liaoning 123000, China;
Key Laboratory of Mine Thermodynamic Disasters &
Control of Ministry of Education,
Fuxin Liaoning 123000, China,
Gang Bai,
College of Safety Science & Engineering,
Liaoning Technical University,
Fuxin Liaoning 123000, China,
Key Laboratory of Mine Thermodynamic Disasters &
Control of Ministry of Education,
Fuxin Liaoning 123000, China,
M e-mail: baigang_1992@163.com
Конфликт интересов
Авторы заявляют об отсутствии конфликта интересов.
Все авторы прочитали и одобрили окончательный вариант рукописи.
Сведения об авторах
Сюэмин Ли,
Колледж безопасности и инжиниринга, Ляонинский технический университет, 123000, Фусинь Ляонин, Китай; Лаборатория горных термодинамических аварий и контроля Министерства образования, 123000, Фусинь Ляонин, Китай.
Ганг Бай,
Колледж безопасности и инжиниринга, Ляонинский технический университет, 123000, Фусинь Ляонин, Китай; Лаборатория горных термодинамических аварий и контроля Министерства образования, 123000, Фусинь Ляонин, Китай, И e-mail:baigang_1992@163.com
