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12БЕЗОПАСНОСТЬ ДЕЯТЕЛЬНОСТИ ЧЕЛОВЕКА HUMAN LIFE SAFETY
Original article / Оригинальная статья УДК 54.064
DOI: https://doi.org/10.21285/2500-1582-2019-3-290-305
Gas generation law of lignite combustion under different air volumes
© Gang Bai1, Xueming Li2, Xihua Zhou3, Jiren Wang4, Jianshe Linghu5
1,2,3,4College of Safety Science & Engineering, Liaoning Technical University, Fuxin Liaoning, China
1 5
, Postdoctoral Research Workstation of Yangquan Coal Industry (group) Co.,Ltd, Yangquan Shanxi, China 1,2,3,4Key Laboratory of Mine Thermodynamic Disasters & Control of Ministry of Education, Fuxin Liaoning, China
Abstract: Spontaneous combustion, if not properly controlled in the process of closure, is very prone to secondary disasters. It is necessary to prevent gas explosion accidents. The TG/DSC technology and the tube furnace temperature-programmed experimental system were used to investigate the evolution of coal mass, heat effect and gas generation laws. The results show that the temperature range of combustion is 247°C-441°C. As the air volume increases, the amount of gas generation decreases, the overall lag effect is presented and the initial temperature of gas lags. At the same temperature, a larger air supply indicates smaller gas generation. The production of CO, CO2, CH4, C2H6, C3H8, C2H4 and C2H2 gases increases with the temperature when the amount of coal is sufficient. CO2, C2H6, C3H8, C2H4 and C2H2 gases reach maximum generating temperatures that correspond to CO and CH4 gas lag. At the same temperature, as the air volume increases, the volume fraction of O2 increases and the production of CO, CO2, CH4, C2H6, C3H8, C2H4 and C2H2 gases decreases. At the combustion stage, an exponential relationship exists between the generation of CO, CH4, C2H6, C3H8 and C2H4 and the volume of air supply, and a linear relationship exists between CO2, C2H2 and the volume of air supply. A smaller air supply volume indicates a large volume of flammable gas and considerable risk of gas explosion.
Keywords: lignite combustion, temperature point, air volume, hysteresis effect, apparition temperature, combustion state, explosion hazard
Information about the article: Received March 20, 2019; accepted for publication June 27, 2019; available online September 30, 2019.
Acknowledgements: This work was financially supported by the National Natural Science Foundation of China (Grant No. 51274115). These supports are gratefully acknowledged. The authors are grateful to the reviewers for discerning comments on this paper.
For citation: Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu. Gas generation law of lignite combustion under different air volumes. XXI century. Technosphere Safety. 2019;4(3):290-305. (In Russian). DOI: 10.21285/25001582-2019-3-290-305.
Исследование законов газообразования при горении лигнита и подаче разного объема воздуха
Ганг Бай1, Сюэмин Ли2, Сихуа Чжоу3, Цзижень Ван4, Цзяньшэ Линху5
1,2,3,4Колледж безопасности и инжиниринга, Ляонинский технический университет, Фусинь Ляонин, Китай 1,5Исследовательская станция, Янцюань Коал Индастри, Янцюань Шаньси, Китай
1,2,3,4Лаборатория горных термодинамических аварий и контроля Министерства образования, Фусинь Ляонин, Китай
Резюме: Неконтролируемое самовозгорание может вызвать вторичные аварии. Необходимо предотвращать взрывы газа. Технология TG / DSC и экспериментальная система с программируемой температурой использовались для исследования изменения массы угля, теплового эффекта и законов газообразования. Результаты показали, что температурный интервал сгорания составляет 247-441°C. По мере увеличения объема воздуха количество выделяемого газа уменьшается, отмечается запаздывание начальной температуры. При одинаковой температуре увеличение подачи воздуха указывает на сокращение объема газообразования. Объем CO, CO2, CH4,
Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu. Gas generation law of lignite
combustion under different air volumes Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху. Исследование законов газообразования при горении лигнита и подаче разного объема воздуха
С2Н6, C3H8, С2Н4 и С2Н2 увеличивается по мере увеличения температуры, если количество угля является достаточным. С02, С2Н6, C3H8, С2Н4 и С2Н2 достигают максимальных температур. При одинаковой температуре по мере увеличения объема воздуха увеличивается объем 02 и уменьшается объем СО, С02, СН4, С2Н6, С3Н8, С2Н4 и С2Н2. На стадии сгорания наблюдается экспоненциальная зависимость между объемом СО, СН 4, С2Н6, С3Н8 и С2Н4 и объемом подачи воздуха. Отмечается линейная зависимость между объемом СО2, С2Н2 и объемом подачи воздуха. Меньший объем подачи воздуха указывает на больший объем горючего газа и возможность взрыва.
Ключевые слова: сжигание лигнита, температурная точка, объем воздуха, эффект гистерезиса, температура, состояние горения, опасность взрыва
Информация о статье: Дата поступления 20 марта 2019 г.; дата принятия к печати 27 июня 2019 г.; дата онлайн-размещения 30 сентября 2019 г.
Благодарности: Работа выполнена при финансовой поддержке Национального фонда естественных наук Китая (грант № 51274115). Эти поддержки с благодарностью признаны. Авторы благодарны рецензентам за проницательные комментарии к этой статье.
Формат цитирования: Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху. Исследование законов газообразования при горении лигнита и подаче разного объема воздуха. XXI век. Техносферная безопасность. 2019;4(2):290—305. РО!: 10.21285/2500-1582-2019-3-290-305.
1. Introduction
Coal will remain the main energy source especially in developing countries, such as China, India and South Africa [1-8]. Spontaneous combustion of coal is a complex process; in China, fire caused by spontaneous combustion accounts for 85%-90% of the total number of mine fires [9-10]. Since 1949, 25 major accidents occurred; fatalities exceeded 100 people in coal mines; 3954 people died. There were 19 accidents of gas (coal dust) explosion (accounting for 76%) as a result of which 2859 people died (accounting for 72.3%); 1 fire accident as a result of which 110 people died (accounting for 2.78%). In fire zones, disturbance factors (e.g., changes in the air volume) affect the concentration and distribution of detonating gas (e.g., CH4), which may cause secondary accidents, such as fire and gas explosions. For example, on March 29, 2013, a gas explosion occurred during the sealing of spontaneous combustion in the goaf area of Tonghua and Jilin Mining Groups in China - 53 people died [11]. On June 23, 2014, the spontaneous combustion of coal triggered a gas explosion in a mining area that killed 22 people in Chongqing, China [12]. Undoubtedly, feasible control of the spontaneous combustion of coal remains a crucial
problem [13-18].
When a mine fire breaks out, coal is at the stage of high-temperature combustion; flammable and explosive gas is generated, and the high temperature in the fire area changes the limits of gas explosion and increases its risk [19]. At present, mine fire prevention and control mainly focuses on coal oxidation at the low temperature oxidation stage, the relationship between index gas and temperature change and influence factors of spontaneous combustion of coal and calculation of its characteristic parameters [20-27]. During a mine fire, the actual temperature and gas distribution of coal in the fire area directly affect the emergency rescue and decision of the accident. Generally, when fire occurs in a coal mine, sealing the fire area is the main treatment measure. However, the secondary gas explosion accident induced by reduced air supply occurs easily during fire area closure, especially in gas mines. During the fire area closure, the air supply is reduced and the concentration of O2 is gradually reduced because of an increase in the coal consumption rate, thereby affecting the distribution of temperature and gas concentration fields in the fire area. When the fire area is certain, the type, concentration and temperature of gas produced by coal combustion have a certain
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regularity. Therefore, the combustion state of the fire area can be predicted by analysing the gas composition and the relationship between gas concentration and temperature changes on the basis of test conditions of the mine. Therefore, it is necessary to investigate the law of gas generation under different conditions of coal combustion and heat release.
In this work, lignite in the No. 4 coal seam of the Rui'an Coal Mine of Inner Mongolia Pingzhuang Coal Group Ltd. was used as a research object. A NETZSCH-STA449C synchronous thermal analyser was used to study the variations of quality, temperature points and heat release rates of lignite burning at 40 mLmin-1 and dividing the phase of its combustion. The gas generation rule of lignite combustion when the air supply volume is 40, 80, 120, 160 and 200 mLmin-1 was studied by using the tube furnace temperature-programmed experiment. The study can be applied to determine the combustion state of the fire area, the temperature of the fire zone and the change of gas in the process of sealing fire areas, which provides theoretical support for preventing gas explosion accidents caused by sealing fire area.
2. Experiments and method
Selection and preparation of samples. The coal samples were obtained from Rui'an Coal Mine in the east of the Inner Mongolia Autonomous Region, China, that frequently suffered spontaneous combustion during coal production. Coal samples collected in the laboratory were smashed with swaying highspeed universal grinder, sifting coal samples
between 50-80 mesh and sealed and preserved in 60 mL Brown wide mouth bottles. The results of the proximate analyses of samples based on the Chinese standard GB/T 30732-2014 are shown in Tab. 1.
Thermal dynamic experiment of coal burning. The experiment uses coal quality as an inves-tigation parameter. Heating and oxidising experiments were conducted on the basis of the heating rate programmed by the coal sample. The change of the quality of the coal sample was measured. The thermogravimetric (TG) curve consisted of the change of the quality of the coal sample at different temperatures. The experimental conditions were as follows: coal sample mass of 15 mg; heating rate of 5°Cmin-1; experimental purge gas (reactive gas) using air; inlet gas flow of 40 mLmin-1; and temperature range of 20°C-800°C.
A German NETZSCH-STA449C synchronous thermal analyser was used in the experiment. TG analysis was performed on the coal samples under temperature-programmed conditions. The instrument device is shown in Fig. 1, which adopts the top-loading structure and is easy to operate. The furnace body was designed with a vacuum seal. When the furnace body was opened, the sample support was separated from the weighing system, which was conducive to the protection of the weighing system. The atmosphere naturally flowed upwards and downwards; thus, the decomposition product could be carried away with only a small amount of flow, and a standard three-way gas mass flowmeter was provided.
Analysis results of the coal industry Результаты анализа угольной промышленности
Table 1
Таблица 1
Coal sample Mad, % Aad, % Vad, % FCad, %
Rui'an coal 24.820 4.705 27.865 42.61
Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu. Gas generation law of lignite
combustion under different air volumes Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху. Исследование законов газообразования при горении лигнита и подаче разного объема воздуха
igram control sys Balance
i Sensor
(Differential
(transformer)
Counterweight
b
Fig. 1. NETZSCH-STA449C synchronous thermal analyser. a - device diagram of synchronous thermal analyser; b - schematic of the synchronous thermal analyser
Рис. 1. Синхронный термический анализатор NETZSCH-STA449C. а - схема устройства синхронного термического анализатора; б - схема синхронного термического анализатора
Experimental system of temperature-programmed furnace and experimental conditions. The experimental system was mainly composed of the following parts: (1) gas supply system, including high-purity synthetic air, gas cylinders, reducing valve, glass rotameters and ventilation pipeline; (2) heating system, including double-tube electric stove, porcelain boat and ventilation pipeline; (3) temperature testing system, including K-type
thermocouple and temperature display; and (4) gas analysis system, including gas sampling bladder and GC-4085 gas chromatograph. When the quality of the coal sample was less, the temperature of the tubular furnace was the same as that of the ceramic boat. The test conditions are shown in Tab. 2, and the principle of the experimental device is shown in Fig. 2.
Experimental conditions Условия эксперимента
Table 2
Таблица 2
Coal sample quality, g Heating rate, °Cmin-1 Air flow, mLmin-1 Gas pressure of cylinder outlet, Mpa
2 2 40, 80, 120, 160, 200 0.1
Fig. 2. Structure of the experimental device Рис. 2. Структура экспериментального устройства
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3. Results and discussion
Thermal dynamic parameter analysis of spontaneous coal combustion. The TG analysis of Rui'an lignite was conducted using a synchronous thermal analyser. TG-Differential Scanning Calo-rimetry (DSC) curves of different lignite supplies were obtained using a TG test (Fig. 3).
As shown in Fig. 3, the spontaneous combustion of coal can be divided into four stages: de-hydration and desorption (Stage I), oxidation (Stage II), combustion (Stage III) and burnout (Stage IV).
The temperature range in Stage I is 20°C-117°C. At this stage, as the temperature increases, the coal samples begin to lose moisture and the water loss rate reaches the maximum value at 65°C. The mass loss is large at approximately 24.3%. In Stage II, the temperature range is 117°C-247°C. At this stage, coal samples begin to react with O2
Dehydration and Desorption stage
/ Oxidation stage Combustion stage
and the mass begins to increase. The increase rate in O2 gain at approximately 190°C reaches a maximum, and the increased mass is approximately 0.4%. The temperature range in Stage III is approximately 247°C-441°C. At this stage, coal samples begin to undergo a combustion reaction and the reaction is most intense at 408°C; the loss mass is approximately 70.63%. At 441°C, quality of the coal sample is constant. The sample turns into ash, and the residual ash is 7.48%.
The heat change of lignite combustion in the temperature increase can be divided into two stages, namely, water evaporation and endothermic stage and exothermic stage of coal oxidation (including Stages II and III). The characteristic temperature points of coal combustion are shown in Tab. 3. The temperature is integrated into Stage I to Stage III heat release in Tab. 4.
100 -90807060-
I 50- 403020100
0
24 22 20 18 16
14^ 12 E1 10 S 8 t
с ° 6 и
4 Q
2
0
-2
800
Temperature/°C
Fig. 3. TG-DSC curves of different metamorphic degree coals Рис. 3. TG-DSC кривые угля разной степени метаморфизма
Temperature points at various stages of coal combustion Температура на разных стадиях сжигания угля
Table 3 Таблица 3
Temperature Temperature, °C
Maximum water loss rate 65
Starting point of O2 absorption 117
Maximum O2 uptake rate 190
Ignition temperature 247
Maximum combustion rate 408
Burnout temperature 536
100
200
300
400
500
600
700
Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu. Gas generation law of lignite
combustion under different air volumes Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху. Исследование законов газообразования при горении лигнита и подаче разного объема воздуха
Released heat during the spontaneous combustion of coal
Table 4 Таблица 4
Тепло, выделяемое при самовозгорании угля
Stage I, (Jg-1) Stage II, (Jg-1) Stage III, (Jg-1) Initial exothermic temperature, °C
-49.62 124.16 1329.13 96.5
In conclusion, the combustion stage of Rui'an lignite is at 247°C-441°C. This work focuses on gas generation at the coal combustion stage; thus, the heating test analyses changes of O2, CO, CO2, CH4, C2H6, CaHs, C2H4 and C2H2 gas products at 40°C-450°C.
Analysis of the initial temperature of gas and the volume fraction in lignite combustion. In the temperature-programmed test of the tubular furnace, the combustion temperature is collected at every temperature interval in the range 40°C-450°C, and the gas sample bags are loaded and analysed by a gas chromatograph. The initial temperature curve of CO, CH4, C2H6, CaHs, C2H4 and C2H2 (Fig. 4) and the relationship curve of the variation of air volume were obtained (Fig. 7-8). The gas generation temperature and production amounts under different air volume are com-
pared, and the influence of air volume on gas generation is analyses. The experimental results using the initial appearance temperatures of gas under different air volumes (the initial appearance temperature is the corresponding temperature when the volume fraction of gas is first detected) are shown in Fig. 4.
As shown in Fig. 4, CO and CH4 gases can be detected at a low temperature, mainly because the internal adsorption of some CH4 gas and the reaction temperatures of coal and O2 are low. Then, C2H4 and C2H6 gases are detected; the occurrence temperature of C3Hs gas lags behind those of C2H4 and C2H6. The initial temperature of C2H2 gas is 250°C. The production of gas produced by lignite is considerably influenced by air volume, and the initial temperature of gas is different under
260 240 220
0 200
2 180 3
3 160
и a
S 140
1 120
1 100
и
80
о
60 40
С2Н2
С3Н8
С2Н6
С2Н4
CO and CH.
20 40 60 80 100 120 140 160 180 200 220 Air volume ( mL ■ min-1 )
Fig. 4. Initial temperature of hydrocarbons under different air volumes Рис. 4. Начальная температура углеводородов при разных объемах воздуха
БЕЗОПАСНОСТЬ ДЕЯТЕЛЬНОСТИ ЧЕЛОВЕКА HUMAN LIFE SAFETY
different air volumes. The effect of air volume is generally lagging; that is, a larger air volume indicates a higher initial temperature of gas and later generation time, and vice versa. The primary reason for these phenomena is that the amount of air is extremely large and the heat produced by coal burning is difficult to accumulate, thereby resulting in the reducing of gas formation rate. The rate of body formation is reduced. These results indicate that the high volume of air inhibits coal oxidation and combustion and the formation of CO, CH4, C2H6, C3H8, C2H4 and C2H2 gases.
Law of gas generation in combustion process. Variation of the volume fraction of O2 under different air volumes. The volume fraction of O2 under different air volumes is shown in Fig. 5. The volume fraction of O2 in the process of programmed heating is extremely small and decreases slowly during heating up at 40°C-200°C. The spontaneous combustion
of coal is at the stage of preheating and losing water. The volume fraction of O2 in coal decreases rapidly at 200°C-400°C, lignite undergoes a severe oxidation reaction and combustion occurs. Furthermore, the volume fraction of O2 in coal reaches the minimum value at 400°C. At 400°C-450°C, the volume fraction of O2 in coal begins to increase due to insufficient coal. A smaller air volume implies a larger decline in the volume fraction of O2 and greater O2 consumption. As the air volume increases, the O2 consumption in coal combustion becomes less and the change trend of the volume fraction of O2 becomes slower. To determine the influence of air volume on coal oxidation and O2 consumption, the air volume is used as the abscissa and the volume fraction of O2 is used as the ordinate. The change curve of the air volume and the volume fraction of O2 at the same temperature is obtained
(Fig. 6).
22 20 18 16
1 14 112
<И
Ю 10
J 8
o" 6 4 2 0
—•— 200 mLmin-160 mLmin-—T— 120 mLmin-—80 mLmin-1 —*— 40 mLmin-1
50 100 150 200 250 300 350 400 450 500
Temperature/°C
Fig. 5. Volume fraction of O2 under different air volumes Рис. 5. Доля О2 при разных объемах воздуха
20 40 60 80 100 120 140 160 180 200 220 Air volume/( mLmin-1)
Fig. 6. Relationship of volume fraction of O2 and air volumes at the same temperature Рис. 6. Соотношение О2 и объемов воздуха при одинаковой температуре
0
2
Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu. Gas generation law of lignite
combustion under different air volumes Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху. Исследование законов газообразования при горении лигнита и подаче разного объема воздуха
As the air volume increases, the volume fraction of O2 shows an increasing trend. At the stage of low-temperature oxidation (before 250°C), the change trend of the volume fraction of O2 with the air volume is small. When coal is at the combustion stage (after 250°C), the change trend of the volume fraction of O2 is evident and the O2 consumption and its rate increase. At the same temperature, the volume fraction of O2 increases with the air volume. The primary reason for this phenomenon can be attributed to the large air volume; generated heat is difficult to accumulate by coal combustion, coal-O2 composite reaction rate decreases and O2 consumption reduces. As the temperature rises, the volume fraction of O2 decreases, the volume fraction of O2 at 400°C is the smallest, and the O2 consumption is maximum; these results are consistent with the TG curve reaching a rapid combustion state at approximately 408°C. The volume fraction of O2 at 450°C is greater than that at 400°C, which is mainly because after 433°C, the coal quantity is insufficient to enter the burnout stage.
Variations of volumes of CO and CO2 under different air volumes. The volume frac-
tions of CO and CO2 under different air volumes are shown in Fig. 7.
At 40 C-200°C, the lignite has a slow oxidation reaction, the volume fractions of the generated CO and CO2 under different air supply volumes are relatively low and the change rule is not evident (Fig. 7). Coals undergo severe oxidation from 200°C to 348°C in the combustion stage; the TG curve indicates that the combustion reaction of coal occurs at 200°C-400°C. The volume fractions of the generated CO and CO2 sharply increases. At 348°C-450°C, the combustion state of lignite begins to weaken and the volume fractions of CO and CO2 sharply decrease. The volume fractions of CO produced by lignite under different air volumes at the same temperature vary. The volume fractions of the generated CO and CO2 decrease with the increase in air volume. The analysis of the experimental data indicates that the volume fractions of the generated CO and CO2 in the oxidation and combustion stages of coal change are exponentially related to the temperature. The volume fractions of CO and CO2 decrease with the increase in air volume under the same temperature at the lignite combustion stage.
5 г
I
2
>
О О
1
-♦— 200mL-min'1 -▼— 160mL-min'1 -•— 120mL-min'1 — 80mL-min'1 -■— 40mLmin'1
50 100 150 200 250 300
Temperature/°C
a
350 400 450
40
35
30
25
S 20
■o
^15
О
О
10 5 0
—200mL-min —▼— 160mLmin —•- 120mLmin —A— 80mL-min'1 —■— 40mLmin'1
50 100 150 200 250 300 Temperature/°C b
350 400 450
Fig. 7. The change law for the volume fraction of CO and CO2 under different air volumes. a - volume fractions of CO; b - volume fractions of CO2
Рис. 7. Закон изменения объемной доли СО и СО2 при разных объемах воздуха.
а - доля СО; б - доля СО2
4
3
0
0
0
БЕЗОПАСНОСТЬ ДЕЯТЕЛЬНОСТИ ЧЕЛОВЕКА HUMAN LIFE SAFETY
A comparison of Fig. 7, a and 7, b shows that the volume fraction of CO2 is considerably larger than that of CO. This result can be mainly attributed to the fact that as the temperature increases, the conversion rate of ordinary molecules of coal into active molecules increases, the generated transition complexes of CO and CO2 increase, Among them, the growth rate of the transition complex generating CO2 is greater than that of the transition complex generating CO. The volume fraction of CO initially increases with the increase in temperature. When coal samples reach a rapid combustion state, the volume fraction of CO decreases slightly, whereas the that of CO2 continues to increase; this result is mainly due to the full combustion of coal samples, which results in a relative decrease in the formation of CO.
Change law for hydrocarbons under different air volumes. The change of gas volume fraction of CH4, C2H6 and C3H8, C2H4 and C2H2 under different air volumes is shown in Fig. 8. As shown in Fig. 8, a, the slow oxidation reaction occurred at 40°C-250°C coal; to generate CH4, gas volume fraction must be low. A sharp rise in the CH4 gas produced by coal combustion at 250°C-348°C, due to the weakening of coal combustion state, the volume fraction of CH4 gas decreased sharply at 348°C-438°C. As shown in Fig. 8, b and 8, c, C2H6 and C3H8 gases increased slowly at 120°C-200°C. In the range 200°C-348°C, the oxidation of coal increased and entered the combustion stage, and the C2H6 and C3H8 gases increased rapidly. At 348°C-438°C, C2H6 and C3H8 gases produced by coal combustion decreased sharply, and the combustion state of lignite began to weaken at this stage. As shown in Fig. 8, d, at 100°C-150°C, the coal slowly oxidised and the C2H4 gas volume fraction was low. At 150°C-399°C, the coal undergoes a violent oxidation reaction and enters the combustion stage, and the volume fraction of C2H4 gas rises sharply. This result occurs mainly because the fatty hydrocarbon in the free phase in the coal and the fat side
chain in the aromatic ring gradually split into C2H4 gas by free radicals as temperature rises. At 399°C-438°C, the C2H4 concentration decreased because of the low combustion of coal. As shown in Fig. 8, e, below 200°C, the coal is in the low-temperature oxidation stage and begins to generate C2H2 gas. When the coal enters the combustion stage (200°C-399°C), the volume fraction of C2H2 gas increases. Coal combustion decreases at 400°C-438°C, and the volume fraction of the generated C2H2 gas drops sharply.
Relationship between gas generation law and air volume at the combustion stage. At the low-temperature oxidation stage, the amount of gas generated by coal oxidation is small and the variation rule is not evident (Fig. 7-8). Thus, the variation rule of gas at the combustion stage is investigated. The variation curve of gas production and air volumes under the same temperature is shown in Fig. 9.
As shown in Fig. 9, the volume fractions of CH4, C2H6, C3H8, C2H4, C2H2, CO and CO2 gases generated by the combustion of lignite decrease with an increase in the air volume. At the same temperature, production of CH4, C2H6, C3H8, C2H4, C2H2, CO and CO2 decreases with an increase in the air volume. The main reason for these results is that when the air volume is large, the heat generated is not easy to accumulate, and the dilution effect of the air flow on gas is crucial; the coal O2 compound reaction rate decreases and the amount of gas production reduces. The production of CH4 and CO gases reaches the highest value at 348°C, and the production of CO2, C2H6, C3H8, C2H4 and C2H2 gases reaches the highest value at 400°C. The decrease in gas production at 450°C is due to the fact that the coal has already entered the burnout stage after 433°C with insufficient coal. By fitting the test data, the volume fractions of CH4, C2H6, C3H8 and C2H4 change exponentially with the air volume at the combustion stage of lignite, and the volume fraction of C2H2 has a linear relationship with the air volume.
Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu. Gas generation law of lignite
combustion under different air volumes Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху. Исследование законов газообразования при горении лигнита и подаче разного объема воздуха
0.30
0.28
0.26
0.24
0х 0 22
^
О 0.20
О 0.18
<¿3 d) 0.16
N 0.14
о > 0.12
üf 0.10
и 0.08
0.06
0.04
0.02
0.00
—200mL ■ min"1 —▼— 160mL^min-1 —А— 120mL^min-1 —•— 80mL-min"1 —■— 40mL^min-1
0.04
50 100 150 200 250 300 Temperature/°C
350 400 450
—200mL-min —A— 160mL-min —■— 120mL-min —•— 80mL^min-1 —■— 40mL-min"1
50
100 150 200 250 300 350 400 450 Temperature/°C
0.006 г
0.005
0.004
щ 0.003
0.002
0.001
0.000
— 200mL-min"1 -T— 160mL^min-1
120mL-min"1
— 80mL^min-1
— 40mL-min"1
0 50 100 150 200 250 300 350 400 450 Temperature/°C
0.007 0.006 0.005 0.004 0.003 " 0.002 0.001 0.000
200mL-min"1
- 160mLmin"1 120mLmin"1
- 80mLmin"1
- 40mLmin"1
0 50 100 150 200 250 300 350 400 450 Temperature/°C
0.0004
0.0003
0.0002
0.0001
0.0000
200mLmin" —T— 160mLmin" —120mLmin" —A— 80mLmin"1 —■— 40mLmin"1
50 100
150 200 250 300 350 400 450 Temperature/°C
Fig. 8. Change curves for the volume fraction of saturated hydrocarbons under different air volumes. a - volume fraction of CH4; b - volume fraction of C2H6; c - volume fraction of C3H8; d - volume fraction of C2H4; e - volume fraction of C2H2
Рис. 8. Кривые изменения доли насыщенных углеводородов при разных объемах воздуха.
а - доля СН4; б - доля С2Н6; c - доля C3H8; d - доля C2H4; е - доля C2H2
2019;4(3):290-305
XXI ВЕК. ТЕХНОСФЕРНАЯ БЕЗОПАСНОСТЬ XXI CENTURY. TECHNOSPHERE SAFETY
ISSN 2500-1582
(print) ISSN 2500-1574 (online)
0
0
b
a
d
c
0
e
БЕЗОПАСНОСТЬ ДЕЯТЕЛЬНОСТИ ЧЕЛОВЕКА HUMAN LIFE SAFETY
с^ = 3.18469e-0ar e7e, RR = 0.99761 250°C
0.25 ■
? \ = 10.22569e-00 36,e,R2 =0.99864 • 310°C
= 0.67652e-002 R2 = 0.93071 A 348°C
0.20 " Qo = 0.4151eJ°°165 Q,R2 =0.86419 400°C
. c49 = 0.24551e-0ra RR = 0.97379 ♦ 450°C
20 40 60
80 100 120 140 160 Air volume/С mLmin-1)
180 200 220
Сш = 0.000469e-°'°2594Q, R2 = 0.87774
Сш = 0.16152e™606B, R2 = 0.98796 " 250°C
310 • 310°C
= 0.0611e-002074Q, R2 = 0.92246 A 348°C = 0.08077e-002092Q, R2 = 0.93864
)02133Q, R2 = 0.95562
400°C 450°C
60 80 100 120 140 160 180 200 220 Air volume/С mL-min- )
с ^ 250 = 0.00257e-(,(m5,Q, R2 = 0.87986 0.007 г
с ^ 310 = 0.0057e-0(m4,Q, R2 0.93269
с ^ 348 = 0.00704e-(,(""Q, R2 = 0.98736 0.006
с ^ 400 ^ 450 = 0.00824e-"""IS2Q, R = 0.01566e-0 02943Q, R = 0.99768 = 0.99703 ^o 0.005 1ä .
'I 0.004
<И
I 0.003
"о
>
к* 0.002
о"
0.001 0.000
С250 = 0.00494e-00101'e, R2 = 0.95844 Ст = 0.00588e-°°™e, R2 = 0.95548 С„ = 0.00835e-°°104'e, R2 = 0.93297 С400 = 0.0107e-°'0nI1e, R1 = 0.93881 = 0.01582e-"™"e, R2 = 0.98652
40 60 80 100 120 140 160 180 200 220 Airvolume/С mL-min-1)
20 40 60 80 100 120 140 160 180 200 220 Air volume/С mL-min-1)
4
% о
H 3
40 60 80 100 120 140 160 180 200 220 Air volume/С mL-min-1)
20 40 60 80 100 120 140 160 180 200 220 Air volume/С mL-min-1)
40 p
35 -
30 ■
I 25 ■ ■-d
<8 20 -
115 ■
>
10 о 10
5 0
= 39.2372- 0.20545Q, R = 0.91687
= 43.44808-0.20718Q, R = 0.97372
■ 250°C
• 310°C
i 348°C
t 400°C
♦ 450°C
= 32.54697 - 0.14953Q, R2 = 0.99486
20 40 60 80 100 120 140 160 180 200 220
Air volume/( mL-min- ) g
Fig. 9. Curves of air volume and volume fraction of saturated hydrocarbons at the same temperature. a - volume fraction of CH4; b - volume fraction of C2H6; c - volume fraction of C3H8; d - volume fraction of C2H4; e - volume fraction of C2H2; f - volume fraction of CO; g - volume fraction of CO2 Рис. 9. Кривые объема воздуха и доли насыщенных углеводородов при одинаковой температуре. а - доля СН4; b - доля С2Н6; c - доля C3H8; d - доля C2H4; е - доля С2Н2; f - доля СО; g - доля СО2
300
ISSN 2500-1582
(print) ISSN 2500-1574 (online)
XXI ВЕК. ТЕХНОСФЕРНАЯ БЕЗОПАСНОСТЬ XXI CENTURY. TECHNOSPHERE SAFETY
2019;4(3):290-305
20 40
b
a
20
d
c
? 2
20
e
Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu. Gas generation law of lignite
combustion under different air volumes Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху. Исследование законов газообразования при горении лигнита и подаче разного объема воздуха
At the combustion stage of lignite (250°C-450°C), the relationship between the concentrations of CH4, C2H6, C3H8, C2H4 and CO and the air volume correspond to the curve in Formula (1), and the relationship between concentrations of C2H2 and CO2 and the air volume correspond to the curve in Formula (2):
bQ
C = ae
C = С + dQ ,
(1)
(2)
where Q is the air volume, mLmin; a, b, c and d are the regression coefficients. The regression and correlation coefficients in Formulas (1) and (2) at different temperatures are shown in Table 5.
Table 5 indicates that when coal is sufficient and the temperature increases, the value of re-gression coefficient a of the volume fraction of CO decreases, whereas the value of regression coef-ficient b increases; the value of regression coefficient c of the volume fraction of C2H4 increases, whereas the value of regression coefficient d does not change considerably. Afterwards, the amount of coal decreases. The values of regression coefficients a and c of the volume fractions of CO and C2H4 increase, whereas the values of regression coefficients b and d decrease. The correlation of the fitting curve is high. In summary, after 400°C, the quantity of coal decreases, the values of regression coefficients a and c of the volume fractions of CO and C2H4 increase, the values of regression coefficients b and d decrease and the fitting curve has a high correlation.
Influence of gas generation on the boundary of gas explosion. During fire area closure, the volume of air supply decreases gradually and the amount of gas generation increases. Studies show that the combination of mixed gas changes the explosion limit of
CH4 [19, 28]. From Fig. 7-8, the variations of the maximum concentrations under different air volumes are shown in Fig. 10.
The generation of CO increases with the decrease in air supply decreases, and the maximum volume fraction of CO reaches 5%
_4
when the air volume is 40 mLmin ', as shown in Fig. 10. A volume fraction of CO of 5% at 100°C has been previously obtained. The low explosion limit of CH4 is 2.8%, and the volume fraction of CO gas is 10%. At 100 C, the low explosion limit of CH4 is only 0.7%. At 700 C, the low explosion limit of CH4 is 3.2% [28]. The coal temperature at the combustion stage of fire zone in the enclosed space reaches above 1000°C, which is considerably higher than 100°C. When the volume fraction of CO gas is 5%, the low explosion limit of CH4 is far below 2.8%. At the same time, the amount of generated CO is considerably greater than 5% due to the sufficient amount of coal in the enclosed space. Luo et al. found that after mixing single-component C2H6 and C2H4, the low limit of CH4 explosion decreases [19]. When the volume fractions of C2H6 and C2H4 are 0.375%, the low limit of CH4 decreases to approximately 3.75%. When C2H6 and C2H4 are mixed with CH4, the explosion limit of CH4 decreases. Combustion in the fire zone generates a large number of flammable and explosive gases (e.g., CO, CH4, C2H6, C3H8, C2H4 and C2H2) and inert gases (e.g., CO2). As shown in Fig. 7-9, when the air volume is 40 mLmin-1, the amount of generated CO gas is the largest, followed by C2H6, and C3Hs is equivalent to C2H4. On this basis, When the fire area is closed after the occurrence of inflammable gas mine fire, sealing fire area is an effective control way, but in the process of sealing fire area, the comprehensive consideration of air supply and the quantities of detected volume fraction of CO and C2H6 in the fire area should be provided to prevent gas explosion during fire area closure.
БЕЗОПАСНОСТЬ ДЕЯТЕЛЬНОСТИ ЧЕЛОВЕКА HUMAN LIFE SAFETY
Table 5
Coefficient values of the air volume and volume fractions of CO and C2H4
Таблица 5
Коэффициенты значений объема воздуха и долей CO и C2H4_
Temperature, °C Volume fraction of CO gas, % Volume fraction of C2H4 gas, %
a b Correlation л coefficient R2 c d Correlation л coefficient R2
250 8.84075 -0.02404 0.95918 0.00494 -0.01019 0.95844
310 7.40507 -0.01268 0.94054 0.00588 -0.00897 0.95548
348 6.06306 -0.00643 0.91299 0.00835 -0.01046 0.93297
400 4.5869 -0.00487 0.93364 0.0107 -0.01181 0.93881
450 4.74016 -0.00949 0.95455 0.01582 -0.02896 0.98652
0.000
40 60 80 100 120 140 160 180 200 Air volume/( mLmin-1)
Fig. 10. Curves of maximum volume fractions of gases under different air volumes Рис. 10. Кривые максимальных объемных долей газов при разных объемах воздуха
4. Conclusion
The change in mass and heat discharge in coal combustion was studied using a TG analyser, and the combustion stage was divided. The gas generation law was investigated by heating up the tube furnace. The following conclusions can be drawn.
1. The temperature range of lignite combustion during the weightlessness stage is 20°C-117°C. The temperature range of oxida-
tion weight gain is 117°C-247°C, and that of combustion is 247°C-441 C.
2. As the air volume increases, the amount of gas generation gradually decreases and the overall lag effect and the lag of the initial temperature of gas are evident. At the same temperature, a large amount of air supply implies a small gas generation.
3. Data analysis indicates that when the fuel is sufficient, CO, CO2, CH4, C2H6, C3H8, C2H4 and C2H2 increase, CO2, C2H6,
Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu. Gas generation law of lignite
combustion under different air volumes Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху. Исследование законов газообразования при горении лигнита и подаче разного объема воздуха
C3H8, C2H4 and C2H2 gases correspond with the maximum output, and the quantity that corresponds to the temperature lags behind CO and CH4.
4. During the combustion stage of lignite, the volume fractions of O2 and the generated gases decrease with the increase in air volume at the same temperature. An exponential relationship exists between the generation
quantities of CO, CH4, C2H6, C3H8 and C2H4 gases and the air supply volume. The relationship between the generation quantities of CO2 and C2H2 and the volume of air supply is linear.
5. A smaller air volume indicates greater production of flammable gas and a risk of gas explosion.
References
1. Yu J, Lucas J, Wall T. Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: a review. Progress in Energy and Combustion Science, 2007;33(2):135-170.
DOI: 10.1016/j.pecs.2006.07.003.
2. Li K, Khanna R, Zhang J, Barati M, Liu Z, Xu T et al. Comprehensive Investigation of Various Structural Features of Bituminous Coals Using Advanced Analytical Techniques. Energy & Fuels. 2015;29(11):7178-7189.. DOI: 10.1021/acs.energyfuels.5b02064.
3. Cheng J, Wang X, Si T, Zhou F, Zhou J, Cen K. Pore fractal structures and combustion dynamics of cokes derived from the pyrolysis of typical Chinese power coals. Fuel Processing Technology, 2016;149:49-54. DOI: 10.1016/j.fuproc.2016.04.004.
4. Wang C, Wu S, Lv Q, Liu X, Chen W, Che D. Study on correlations of coal chemical properties based on database of real-time data. Applied Energy. 2017;204(15):1115-1123.
DOI: 10.1016/j.apenergy.2017.03.049.
5. Yi B, Zhang L, Huang F, Xia Z, Mao Z, Ding J et al. Investigating the combustion characteristic temperature of 28 kinds of Chinese coal in oxy-fuel conditions. Energy Convers Manage, 2015;103:439-447.
DOI: 10.1016/j.enconman.2015.06.053.
6. Oboirien B, Thulari V, North B. Enrichment of trace elements in bottom ash from coal oxy-combustion: effect of coal types. Applied Energy, 2016;177:81-86. DOI: 10.1016/j.apenergy.2016.04.118.
7. Zhong W, An H, Fang W, Gao X, Dong D. Features and evolution of international fossil fuel trade network based on value of emergy. Applied Energy, 2016;165:868-877.
DOI: 10.1016/j.apenergy.2015.12.083.
8. Shang Y, Hei P, Lu S, Shang L, Li X, Wei Y et al. China's energy-water nexus: assessing water conservation synergies of the total coal consumption cap strategy until 2050. Applied Energy, 2018;210:643-660.
DOI: 10.1016/j.apenergy.2016.11.008
9. Xiao Yang., LI Qingwei., LU Junhui. Effects of air relative humidity on coal spontaneous combustion
properties [J]. China Safety Science Journal, 2015;3:34-40.
10. Deming, W. Mine Fires; China University of Mining and Technology Press: Xuzhou, 2008, pp. 125-136.
11. Web of Former State Administration of Safety and Production Supervision and Administration. (2013). http://old.chinasafety.gov.cn/jgjc/sgcc/tbzdsgdcbg/2013 07/t20130711_175243.shtml.
12. Web of The Central People's Government of the People's Republic of China. (2014). http://www.gov.cn/xinwen/2014-10/10/content_2762334.htm.
13. Wen H., Zhang F., Jin Y., Liu W. Experiment research on effect of sulfur on characteristic parameters of coal spontaneous combustion. Coal Saf, 2011 ;42:5-7 (in Chinese).
14. Pandey J, Mohalik N, Mishra R, Khalkho A, Kumar D, Singh V. Investigation of the role of fire retardants in preventing spontaneous heating of coal and controlling coal mine fires. Fire Technology. 2012;51(2):227-245. DOI: 10.1007/s10694-012-0302-9.
15. Qi X, Wei C, Li Q, Zhang L. Controlled-release inhibitor for preventing the spontaneous combustion of coal. Natural Hazards, 2016;82(2):891-901.
DOI: 10.1007/s11069-016-2224-1.
16. ZHOU F, WANG D, ZHANG Y, ZHANG Y, LI X. Practice of Fighting Fire and Suppressing Explosion for a Super-Large and Highly Gassy Mine. Journal of China University of Mining and Technology. 2007;17(4):459-463. DOI: 10.1016/S1006-1266(07)60125-4
17. Zhenhua X, Xiaochao L, Mingming L. Application of Three-phase Foam Technology for Spontaneous Combustion Prevention in Longdong Coal Mine. Procedia Engineering. 2011;26:63-69.
DOI: 10.1016/j.proeng.2011.11.2140.
18. Mao Z, Zhu H, Zhao X, Sun J, Wang Q. Experimental Study on Characteristic Parameters of Coal Spontaneous Combustion. Procedia Engineering. 2013;62:1081-1086.
DOI: 10.1016/j.proeng.2013.08.164.
19. Luo Z.M., Lin J.J., Guo Z.C., et al. Influence of vari-
БЕЗОПАСНОСТЬ ДЕЯТЕЛЬНОСТИ ЧЕЛОВЕКА HUMAN LIFE SAFETY
ous other flammable gases in coal mine on explosion limits of methane [J]. China Safety Science Journal, 2015;1:91-97.
20. Avila C, Wu T, Lester E. Estimating the Spontaneous Combustion Potential of Coals Using Thermograv-imetric Analysis. Energy & Fuels. 2014;28(3):1765-1773. DOI: 10.1021/ef402119f.
21. Zhang Y, Li Y, Huang Y, Li S, Wang W. Characteristics of mass, heat and gaseous products during coal spontaneous combustion using TG/DSC-FTIR technology. Journal of Thermal Analysis and Calorimetry. 2017;131(3):2963-2974. DOI: 10.1007/s10973-017-6738-x.
22. Baris K, Kizgut S, Didari V. Low-temperature oxidation of some Turkish coals. Fuel. 2012;93:423-432. DOI: 10.1016/j.fuel.2011.08.066.
23. Zhang W, Jiang S, Wang K, Wang L, Xu Y, Wu Z et al. Thermogravimetric Dynamics and FTIR Analysis on Oxidation Properties of Low-Rank Coal at Low and Moderate Temperatures. International Journal of Coal Preparation and Utilization. 2014;35(1):39-50.
DOI: 10.1080/19392699.2013.873421.
24. Zhang Y, Wang J, Wu J, Xue S, Li Z, Chang L. Modes and kinetics of CO2 and CO production from low-temperature oxidation of coal. International Journal of Coal Geology. 2015;140:1-8.
DOI: 10.1016/j.coal.2015.01.001.
25. LI Wei. Experimental research on the rules of the characteristic parameters of coal spontaneous combustion [D]. Xi'an: Xi'an University of Science and Technology, 2008.
26. Qin Y.P., Xu S.M., Qiao T, Li Y.Q., Influence of air flow on spontaneous combustion characteristics of residual coal. Safety in Coal Mines, 2014,45(11 ):34-37.
27. Zhou F.B., Shao H., Li J.H., et al. Experimental research on combustion product formation during coal spontaneous combustion under reduced oxygen concentrations [J]. Journal of China University of Mining & Technology, 2010;39(6):808-812.
28. Zhou X.H., Meng L., Shi M.J., Guo L.H., Zhao J.Y., Feng C.C. Influences of sealing fire zone in high gas mine on impact factors of gas explosion limits. Explosion and Shock Waves, 2013,33(4):351-356.
Contribution
Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu have equal authors' rights and responsibility for plagiarism.
Conflict of interests
The authors declare no conflict of interests.
Information about the authors
Gang Bai,
College of Safety Science & Engineering,
Liaoning Technical University,
Fuxin Liaoning 123000, China,
Postdoctoral Research Workstation of Yangquan Coal
Industry (group) Co.,Ltd,
Yangquan Shanxi ,045000, China
Key Laboratory of Mine Thermodynamic Disasters &
Control of Ministry of Education,
Fuxin Liaoning 123000, China,
El e-mail: [email protected]
Xueming Li,
College of Safety Science & Engineering, Liaoning Technical University, Fuxin Liaoning 123000, China.
Критерии авторства
Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху имеют равные авторские права и несут равную ответственность за плагиат.
Конфликт интересов
Авторы заявляют об отсутствии конфликта интересов.
Сведения об авторах
Ганг Бай,
Колледж безопасности и инжиниринга, Ляонинский технический университет, 123000, Фусинь Ляонин, Китай; Исследовательская станция, Янцюань Коал Индастри, 045000, Янцюань Шаньси, Китай; Лаборатория горных термодинамических аварий и контроля Министерства образования, 123000, Фусинь Ляонин, Китай, И е-таН;Ьа1дапд_1992@163.сот 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.
Gang Bai, Xueming Li, Xihua Zhou, Jiren Wang, Jianshe Linghu. Gas generation law of lignite
combustion under different air volumes Ганг Бай, Сюэмин Ли, Сихуа Чжоу, Цзижень Ван, Цзяньшэ Линху. Исследование законов газообразования при горении лигнита и подаче разного объема воздуха
Xihua Zhou,
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.
Jiren Wang,
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. Jianshe Linghu,
Postdoctoral Research Workstation of Yangquan Coal Industry (group) Co.,Ltd, Yangquan Shanxi ,045000, China.
Сихуа Чжоу,
Колледж безопасности и инжиниринга, Ляонинский технический университет, 123000, Фусинь Ляонин, Китай; Лаборатория горных термодинамических аварий и контроля Министерства образования, 123000, Фусинь Ляонин, Китай. Цзижень Ван,
Колледж безопасности и инжиниринга,
Ляонинский технический университет,
123000, Фусинь Ляонин, Китай;
Лаборатория горных термодинамических аварий
и контроля Министерства образования,
123000, Фусинь Ляонин, Китай.
Цзяньшэ Линху,
Исследовательская станция,
Янцюань Коал Индастри,
045000, Янцюань Шаньси, Китай.
