Coal oxidation with air stream of varying oxygen content and flow rate - Fire gas emission profile
Introduction
An underground fire constitutes one of the most serious threats in the mining industry, often requiring long-term rescue operations [[1], [2], [3], [4]]. This is primarily due to the specificity of underground mining excavations in which the limited capacity causes rapid excess of the permissible concentration of gases harmful to human health, and makes the escape of employees due to smoke and limited amount of oxygen difficult. The undertaken fire-fighting activities usually result in the partial suspension of mining works in the mine, and in the case of passive fire extinguishing it is necessary to dam up the fields for their isolation. As a result of these actions, the fire field devices and unused coal resources are often abandoned. Besides, endogenous fires, especially in wall goafs, can cause the ignition and explosion of methane resulting also in coal dust explosions. In Poland, in the period of 2000–2016, about 7% of methane ignition cases were caused by endogenous fires [5,6].
Conducted research on the cause of mine fires has allowed putting forward several theories, such as the theory of oxygen adsorption, the theory of exothermic reaction of pyrite oxidation, bacterial theory, or phenyl theory. However, the theory of oxygen adsorption (oxidation reaction between coal and oxygen molecules) met the greatest acceptance of the scientific community as the main source of coal spontaneous ignition [7]. Coal is a natural porous medium with a diverse pore structure in which micropores, mesopores and macropores predominate. The presence of micropores primarily determines its porosity and gas sorption capacity, while the macro- and mesopores are responsible for coal permeability for the flow of gases [8,9]. The porous structure of coal indicates the possibility of the permeation of oxygen molecules from the air stream through the coal structure. As a result of the contact with oxygen, coal undergoes low-temperature oxidation [10,11]. In the initial stage, the oxidation takes place on the surface of the macropores and near the outer surface of the grains and fissures [12,13]; then this process passes from the outer surface into the coal mass. Oxygen diffuses through the increasingly narrower micro-fissures to the inner surface of the pores where it undergoes chemisorption on the surface active centers [14]. As a result of the oxidation reaction of coal, heat is released, which initiates a slow increase in the temperature of coal from the original temperature of the rock mass to the critical temperature in the range of 60 °C–100 °C [15,16]. Thus, the higher the initial temperature of the coal mass, the faster the critical temperature is gained. Xu [17] investigated the heat release from different types of oxygen containing groups in coal and observed that the decomposition of functional groups constitutes the reason of the slow increase of heat in the initial stage of coal heating. The oxidation of the hydroxyl, carboxyl and carbonyl groups released a small amount of heat and some of the reactions were endothermic. If the heat generated by oxidation is dissipated, the rate of oxidation of the coal surface decreases over time and the temperature decreases, whereas further air supply leads to a weathered state of coal. However, due to the fact that the coals are characterized by a low heat transfer coefficient, in the range of 0.09–0.70 W/m3/K (determined at 25 °C) [18], under certain conditions heat accumulation occurs, which results in coal self-heating and may turn into an uncontrolled fire. Spontaneous combustion occurs when the oxidation intensity and coal temperature start to increase exponentially and the critical temperature is exceeded.
The accumulation of heat emitted during the oxidation reaction initiates the ignition of coal but also the production of a number of fire gases, which affects the variation of mine atmosphere composition. Many studies on thermal coal oxidation have proven that the concentration of the combustion products increases with the increase in coal temperature [[19], [20], [21]]. Carbon monoxide turned out to be the most important gas from the point of view of fire risk assessment [22,23], but hydrogen and hydrocarbons as well [24,25]. Hydrocarbons mainly include three types of gases, i.e. alkanes (propane, ethane and butane), alkenes (propylene and ethylene) and alkyne (acetylene). On the basis of the measured concentration of the gases, the fire indices (e.g. Graham's Ratio, JonesTrickett Ratio, Young Ratio, Morris Ratio, CO/CO2 Ratio, Willet's Ratio and Hydrocarbon Ratio), prescribed in mining regulations, are additionally calculated [23,[25], [26], [27]]. Based on the values of fire indices, fire risk assessment is performed and the temperature of heating coal is forecast.
Many studies have been carried out concerning the interpretation of mine atmosphere during coal spontaneous heating. Guo [21] analyzed the composition of a gas mixture obtained at different temperature points during the heating process of anthracite, lignite and long-flame coal. Test results showed that carbon oxides, methane and ethane were detected at the beginning of the experiment. Ethylene appeared at the temperature of 70 °C in lignite heating process, while in long-flame coal and anthracite heating processes it was detected at a higher temperature (approximately 100–110 °C). Dai [28] also reported that the temperature of ethylene appearance increases with the metamorphic degree. Lu [29], Wu [30] and Dong [31] stated that carbon monoxide plays an important role in fire detection because its concentration increased regularly with the increase in coal temperature. They concluded that at the beginning of the oxidation process, the carbon monoxide concentration rises very slowly, but when the temperature of coal exceeds 100 °C the concentration of gas increases rapidly. The point of intensive gas production changes with the coal rank. They also found that the temperature at which propane, propylene, and acetylene were detected was above 100 °C.
The effect of ventilation rates, oxygen content atmosphere or particle size of coal on oxidation characteristic was also verified in the literature. For example, the influence of oxygen concentration in the air on the spontaneous combustion liability of coal and carbon oxides production have been studied in works by Yuan and Smith [12,32]. They demonstrated that if the inlet oxygen concentration exceeds 15%, the generation of carbon dioxide and carbon monoxide clearly increases. Meanwhile, the test results for different air flow rates showed that at the temperature of 70 °C, the influence of ventilation rate on the amount of carbon monoxide production was poorly visible. Xu et al. [33] tested the propensity of bituminous coal to self-ignition and oxidation under various oxygen conditions. They observed that the coal temperature obtained the oven temperature only with the initial oxygen content between 10.49% and 20.95%. Rambha and Ren [34] studied the conditions that are most favorable for the occurrence of the spontaneous combustion of coal. They found that the coal tested showed the maximum oxidation potential at the airflow rates of 0.2 L/min. Więckowski et al. [35] studied the effect of flow rates on the gas emission profile during coal heating and cooling. The experiment was performed only in supplied oxygen concentration of 20.5%. The author found that the gases concentration and the ratio of CO/CO2 presented a downward tendency with the increased air flow rate. In works [[36], [37], [38]], the authors stated that fire gases are released intensely from the finer fractions and this relationship becomes more visible at higher temperatures.
In Poland, in 1947–1960 the number of endogenous fires in underground coal mines was very high reaching even 600 fires a year in the 1950s. Then, a significant successive decrease in the number of endogenous fires could be observed, e.g. during the last 6 years, their number did not exceed 8 per year, achieving the minimum value in 2014, i.e. only one fire; in 2019 three endogenous fires were recorded [39]. These numbers show that the numerous works performed on various aspects concerning the phenomenon of coal spontaneous heating bring measurable results leading to a significant reduction of the number of fires in underground mining and of the associated fatal accidents.
Based on the review of world literature [[28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]], it was concluded that studies on the self-ignition of coals in the field of fire gas emissions are mostly limited to determining the changes in gas concentration depending on the temperature and generally include such gases as carbon oxides, ethylene, propylene, ethane or propane. Thus, the effect of airflow rate and oxygen concentration on the fire gases emission, particularly hydrocarbons, is poorly understood and provides rationale for the subject matter of this paper. The aim of the present work was to develop a lab-scale experimental set up which allows to characterize the variation of fire gases concentration in different coal heating conditions (e.g. with different air flow: 0.05 L/min and 0.1 L/min as well as different amount of oxygen supplied: 10%, 15% and 21%). The used chromatographs enabled to monitor the changes in the content of carbon monoxide (CO) and carbon dioxide (CO2) as well as in the content of the following hydrocarbons: ethane (C2H6), propane (C3H8), n-butane (n-C4H10), i-butane (i-C4H10), ethylene (C2H4) and propylene (C3H6) even at the level of 0.01 ppm at the outlet of the reactor. Two structural isomers of butane, called normal butane (n-butane) and isobutane (i-butane) are rarely analyzed in the literature as gas indices of coal spontaneous heating. The data from laboratory tests could be potentially valuable for the accurate interpretation of mine atmosphere during self-heating of coal in the field of a coal mine.
Section snippets
Materials
The coal sample tested used in this study originated from one of the coal mines in the region of the Upper Silesian Coal Basin, Poland. The coal is classified within the bituminous coal rank (vitrinite reflectance, Ro = 0.75%). The chemical composition and physical properties of the coal are shown in Table 1. Coal analyses were performed by the accredited laboratory of the Department of Solid Fuels Quality Assessment, Central Mining Institute, Poland.
Carbon, hydrogen and nitrogen contents were
Effect of oxygen content on coal temperature
The effect of the amount of oxygen concentration on coal temperature increase rate is presented in Fig. 3.
At the beginning of the test, the coal temperature is approximately 21 °C and starts to slowly increase with time. In the time range of 0–80 min, the curves are self-overlapping. The temperature differences recorded for the coals oxidized under different oxygen concentrations were very small and equaled about 4–8%. At the time of 80 min, the coal temperature at 21% oxygen content is 40 °C
Conclusions
The results presented in this work prove that the analysis of the composition of mine air constitutes both an effective and useful tool reflecting the status of fire hazard development. Based on precise gas chromatographic analyses, it is possible to determine the temperature of individual coal. Studies have shown that as the temperature increases, the gas stream emitted from the tested carbon sample also increases. It was confirmed that carbon monoxide and ethylene, but also n-butane, called
CRediT authorship contribution statement
Karolina Wojtacha-Rychter: Conceptualization, Methodology, Formal analysis, Investigation, Writing - review & editing. Adam Smoliński: Conceptualization, Methodology, Formal analysis, Investigation, Supervision.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
This work was supported by the Ministry of Science and Higher Education, Poland [grant number 11153010].
References (40)
- et al.
A lab-scale experiment on low-temperature coal oxidation in the context of underground coal fires
Appl. Therm. Eng.
(2018) - et al.
Method for prevention and control of spontaneous combustion of coal seam and its application in mining field
Int. J. Min. Sci. Technol.
(2017) An analysis of underground fires in Polish hard coal mines
Int. J. Min. Sci. Technol.
(2008)- et al.
Characteristics of multi-scale pore structure of coal and its influence on permeability
Nat. Gas. Ind.
(2019) - et al.
Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modeling
Prog. Energy Combust. Sci.
(2003) - et al.
Modes and kinetics of CO2 and CO production from low-temperature oxidation of coal
Int. J. Coal Geol.
(2015) - et al.
CO and CO2 emissions from spontaneous heating of coal under different ventilation rates
Int. J. Coal Geol.
(2011) Heat effect of the oxygen-containing functional groups in coal during spontaneous combustion processes
Adv. Powder Technol.
(2017)- et al.
Data on the analysis of temperature inversion during spontaneous combustion of coal
Data in Brief
(2019) - et al.
Study on primal CO gas generation and emission of coal seam
Int. J. Min. Sci. Technol.
(2017)
Forecasting spontaneous combustion of coal in underground coal mines by index gases: a review
J. Loss Prev. Process. Ind.
Early detection of spontaneous combustion of coal in underground coal mines with the development of an ethylene enriching system
Int. J. Coal Geol.
Mine fire gas indices and their application to Indian underground coal mine fires
Int. J. Coal Geol.
Experimental research on index gas of the coal spontaneous at low-temperature stage
J. Loss Prev. Process. Ind.
Experimental study on significant gases of coal spontaneous combustion by temperature programmed (TP)
Procedia Eng
Law of gas production during coal heating oxidation
Int. J. Min. Sci. Technol.
Experimental study on CO and CO2 emissions from spontaneous heating of coals at varying temperatures and O2 concentrations
J. Loss Prev. Process. Ind.
Spontaneous combustion coal parameters for the crossing-point temperature (CPT) method in a temperature–programmed system (TPS), fire safe
J
Effectiveness of a high-temperature-programmed experimental system in simulating particle size effects on hazardous gas emissions in bituminous coal
Saf. Sci.
An investigation of the factors associated with interpretation of mine atmosphere for spontaneous combustion in coal mines
Fuel Process. Technol.
Cited by (9)
Experimental study of coal spontaneous combustion high-temperature region spreading characteristics
2023, Process Safety and Environmental ProtectionInhibition effect and mechanism of nano-aluminum hydroxide foam on coal spontaneous combustion
2022, Thermochimica ActaCitation Excerpt :This process will be intensified continuously with increasing ambient temperature. Finally, the coal temperature will exceed the ambient temperature at a point, which is called crossing point temperature [31–33]. Crossing point temperature is used as an indicator to evaluate the self-heating and spontaneous combustion tendency of coals.
Characterization of activated carbon precursors prepared by dry-air oxidant and its effects on the adsorptions of activated carbons
2022, FuelCitation Excerpt :As an oxidant, dry air can effectively increase the number of micropores in the material, increase its specific surface area, and make it easier for gas molecules to attach to the surface of the material [27]. The surface structure of the coal can be rapidly changed by passing dry air in a high-temperature furnace with the oxidation temperature set above the critical temperature [28]. Due to the double action of thermal stress and oxidative decomposition, the surface of coal produces a large number of secondary development of fissures and new pores [29,30].
Investigation of macro-kinetics of coal-oxygen reactions under varying oxygen concentrations: Towards the understanding of combustion characteristics in underground coal fires
2022, Process Safety and Environmental ProtectionCitation Excerpt :So far, to the best of the authors’ knowledge, no direct in-situ measurement could verify which interpretation is physically correct. However, what we know is that the reaction zone of UCFs is not constantly exposed to a fixed oxygen concentration (Wojtacha-Rychter and Smoliński, 2020). Therefore, the understanding of the macro-kinetics of coal-oxygen reactions under varying oxygen concentrations would be of both theoretical importance and practical relevance to the efforts in the control and extinguishment of such smoldering fires in UCFs.
The feasibility of CO<inf>2</inf> emission reduction by adsorptive storage on Polish hard coals in the Upper Silesia Coal Basin: An experimental and modeling study of equilibrium, kinetics and thermodynamics
2021, Science of the Total EnvironmentCitation Excerpt :Coal self-ignition takes place after the critical temperature has been exceeded, above this point the oxidation intensity and coal temperature increase exponentially. Simultaneously with the outbreak of the fire, combustible components, i.e. the products of dry distillation (methane, ethylene, acetylene and hydrogen) as well as the products of incomplete combustion of carbon (carbon monoxide) appear in the mine air (Wojtacha-Rychter and Smolinski, 2020). Based on the results presented in Fig. 9, it can be concluded that the temperature increases at the maximum by 6 °C due to the thermal effect of CO2 adsorption; therefore, such an increase of temperature is safe with regard to the majority of Polish coal mines where the primary rock temperature at the depth below 1200 m does not exceed 60 °C (Szlazak et al., 2008).