Coalfield Fire Research Articles

Generation characteristics and molecular reaction dynamics mechanism of PAHs during oxygen-lean combustion of jet coal

Severe coalfield fires occur in China, mainly in the northwest provinces of Xinjiang, Inner Mongolia, and Ningxia. Among the organic pollutants produced by oxygen-lean combustion, PAHs are considered one of the most dangerous pollutants due to their environmental contamination and “carcinogenic, teratogenic, and mutagenic” effects on human beings. In this paper, to address the problems of PAHs generation and reaction mechanism in coalfield fire, the jet coal from Xinjiang Zhundong Coalfield was selected to conduct the experiments, and the combustion products were detected using GC–MS and FTIR. The molecular dynamics method based on the reaction force field was also used to calculate the reaction process of 17 single molecules (5083 atoms) of jet coal combusted from 300 K to 4000 K at 0.5 oxygen equivalent ratio. The experimental results show that the generation of PAHs increased roughly from 573 K to 773 K at the three oxygen concentrations, and an increase in oxygen content can enhance PAHs generation to some degree. The structural evolutions and product changes in the system have been traced, and it is also clarified that the reaction history of oxygen-lean combustion mainly experiences two stages: thermal decomposition of coal structure and transformation of aromatic structure. The generation mechanism of PAHs is proposed from the perspective of molecular reaction dynamics. On the one hand, it is due to the breakage of active sites such as C-S, C-O, and O-H during the decomposition of coal structure. On the other hand, it originates from the polycondensation reaction triggered by the dehydrogenation of aromatic structure that enlarges the aromatic dimension The research results reveal the generation characteristics and reaction pathways of PAHs in the oxygen-lean combustion of low-rank coal in the fire zone and provide certain references for coal fire management and pollutant control.

Estimation and Prediction of Carbon Emission from Typical Coalfield Fire in Xinjiang, China: Based on Field Detection and Historical Data

ABSTRACT Coalfield fires release large amounts of greenhouse gases like CO2 and methane. Quick and effective carbon emission estimates are vital for assessing fire control and enhancing carbon reduction. In this work, field detection was applied to estimate the coal loss of typical coalfield fires in Xinjiang, and a simplified model was used to estimate the carbon emission based on coal loss. This method was verified by comparing with literature data. Using this model, the carbon emissions of Jiangjunmiao, Sigonghe, and Changcaodong coalfield fire zones were estimated to be 1,245,187.5 t, 293,730 t, and 511,122 t, respectively. Besides the field test data, the historical data from government was also collected, in order to calculate the increase rate of carbon emission. In Xinjiang, substantial progress has been made in coal fire management. Our estimation indicates that from 1958 to 2019, Xinjiang’s efforts to control coalfield fires resulted in a reduction of 8.93 Gt of carbon emissions, roughly sevenfold the total carbon emissions of China in 2023. From 2019 to 2023, the annual rates of change in carbon emissions for coal fire areas in Xinjiang that were under active control and those that were left unregulated were −4% and + 13.3%, respectively. In order to achieve the carbon reduction targets in the coalfield fire zone, the three surveyed Xinjiang coalfields need to achieve an annual reduction rate of 58%-73% to achieve the 2030 plan, 20%-29% to achieve the 2050 plan, and 15%-22% to achieve the 2060 plan.

Diffusion of Surface CO2 in Coalfield Fire Areas by Surface Temperature and Wind

Diffusion of Surface CO2 in Coalfield Fire Areas by Surface Temperature and Wind

Microscopic Structural Evolution of Oxidized Coal Residues with Different Oxidation Degree.

Coalfield fires represent a critical environmental and safety concern, warranting a comprehensive understanding of the factors influencing the reactivity of oxidized coal residues within fire zones. This study investigates the influence of the oxygen volume fraction and oxidation temperature on the residual structure of oxidized coal, elucidating the underlying mechanisms driving reduced coal reactivity. The representative oxidation conditions for coalfield fire zones were determined. Through industrial and elemental analyses, complemented by methods such as infrared diffuse reflection, specific surface area determination, and pore size analysis, results indicate that higher temperatures and oxygen levels decrease volatile matter and fixed carbon, notably above 400 °C due to oxygen-deficient combustion. Hydroxyl groups decrease with a rising temperature in high oxygen conditions, while carboxyl groups increase at lower temperatures with elevated oxygen. Oxygen-lean and high-temperature conditions reinforce the coal structure, evidenced by the reduced condensation index in aromatic hydrocarbon. Oxidation alters the pore morphology, progressing from micropores to larger irregular pores through various stages, including pore formation, expansion, and merging. Elevated oxygen levels intensify oxidation, consuming the coal carbon matrix and reducing micropores, hindering internal gas diffusion, which is the key to a reduced coal reactivity in fire zones.

Research on early identification of burning status in a fire area in Xinjiang based on data-driven

The early combustion stage in coalfield fire areas (CFA) is the initial phase of coal fire development and spread, making the effective identification of the combustion state crucial. To effectively identify the early combustion state of CFA, this study investigates the CFA in Xinjiang, China. Based on the abnormal characteristics of shallow soil and surface space, a data-driven evaluation method for identifying early combustion states of CFA is proposed. Construct a soil-gas dual-layer early combustion state assessment model of CFA. The evaluation model was applied and validated in the coal fire areas of Xinjiang, China.The results show that there are obvious differences in the spatial distribution of auto-ignition gas, temperature, humidity, surface vegetation coverage and other indicators in the shallow soil and surface space of the coal fire area. The early combustion states of coal fires in areas A, B, and C in the survey area are all in the combustion stage, the combustion state in area D is in the oxidation stage. The accuracy of the evaluation method is bidirectionally verified by the radon measurement method and the infrared thermal imaging detection method. It provides a theoretical basis for controlling the development and spread of coalfield fire areas.

Exploring the Tar Component Characteristics of Coalfield Fire Residual Coal Under Different Constant Temperature Oxidation

ABSTRACT Due to incomplete combustion of coal, a large amount of tar will be produced in coal fire area, which will cause serious pollution to the environment. In this paper, we dissolve the tar components in the residual coal by solvent extraction technique, analyze the organic components in the residual coal tar at different temperatures by GC-MS, study the characteristics of the residual coal tar components at different temperatures, and analyze the influence of the residual coal tar on the combustion and rekindling characteristics of the fire zone of the coalfield. The results show that: the components of coal combustion residual tar are mainly alkane organics which are easy to be oxidized and volatilized, and the content is more than 55%. At 300°C, the color of residual tar extract is light yellow or yellowish brown, and becomes colorless with the increase of temperature. At 300–500°C, the lower the temperature in the fire zone, the more residual tar in the coal, and the greater the risk of reignition in the extinguishing fire zone. The types of organic compounds in the tar residue from combustion have obvious characteristics at different temperature stages. Organic species have obvious characteristics at different temperature stages. By identifying the temperature stage of the coalfield fire area development, the organic structure characteristics and change trend of the residual coal tar can be deduced, and mastering the organic structure characteristics of the residual tar in the coalfield fire area can also reflect the temperature stage of coal fire area to a certain extent.

Detection and management of coal seam outcrop fire in China: a case study

The outcrop fire area in Rujigou Coal Mine in Ningxia, China has been burning continuously for over 100 years. This not only results in wastage of resources but also poses significant damage to the ecological environment. Previous research on open fire detection has mainly focused on coalfield fire areas, using single method such as infrared remote sensing or surface temperature measurement, magnetic method, electrical method, radon measurement and mercurimetry. However, the outcrop fire area has migrated to deeper parts over the years, conventional single fire zone detection methods are not capable of accurately detecting the extent of the fire zone, inversion interpretation is faced with the problem of many solutions. In fire management, current research focuses on the development of new materials, such as fly ash gel, sodium silicate gel, etc., However, it is often difficult to quickly extinguish outcrop fire areas with a single technique. Considering this status quo, unmanned aerial vehicle (UAV) infrared thermal imaging was employed to initially detect the scope of the outcrop fire area, and then both the spontaneous potential and directional drilling methods were adopted for further scope detection in pursuit of more accurate results. In addition, an applicable fire prevention and extinguishing system was constructed, in which three-phase foam was injected for the purpose of absorbing heat and cooling. Furthermore, the composite colloid was used to plug air leakage channels, and loess was backfilled to avoid re-combustion. The comprehensive detection and control technologies proposed in this study can be applied to eliminating the outcrop fire area and protecting the environment. This study can provide guidance and reference for the treatment of other outcrop fire areas.

Influence of pore-crack environment on heat propagation of underground coalfield fire: A case study in Daquanhu, Xinjiang, China

The development of pores and cracks in coalfield fire area promote the abnormal temperature, the greenhouse effect, and the release of toxic gases. It is of great significance to study the dynamic distribution characteristics of temperature field and concentration field of typical gas products in a coalfield fire area under different pore-crack environments for revealing the spreading law of underground coalfield fire. In this paper, the combined effect of coal combustion chemical reaction, heat energy transmission, flue gas seepage and pore-crack environment were considered. Taking Daquanhu coalfield fire area as an example, an optimized multi-field coupling model, which included the chemical reaction of coal combustion, convection heat transfer in cracks, heat transfer in porous media, gas seepage and improved partial differential equation of pore-crack evolution was constructed. The coalfield fires area was divided into combustion zone, caving zone, sagging subsidence zone, crack zone. The evolution law of temperature field, CO2 concentration field in the coalfield fire area under different air leakage and geological crack conditions was analysed. The results showed that the temperature development of coalfield fire is divided into rapid heating stage and stable development stage. At the rapid heating stage, the greater the air leakage flow was, the faster the maximum temperature rise in the combustion zone. At the stable development stage, with the air leakage decreased, the thermal buoyancy increased. The greater the number of cracks was, the greater the area of the drying zone (673.15 K < T < 1073.15 K) closed to the crack zone. The influence of crack environment on heat flux in the sagging subsidence zone and crack zone was obvious. The total heat flow in the crack zone was about 25 times that in the caving zone. The relationship between the maximum temperature in the combustion zone and the CO2 concentration at the crack outlet showed an exponential model (ExpGro1 exponential model). The area spread by coalfield fire increased linearly with the rang of surface temperature abnormal. The research results provided a basis for judging the spread trend of coalfield fire and extracting coalfield fire of heat energy.

Oxygen-Lean Combustion Law of Briquetted Coal Under Stress in Coal Field Fire Zone

ABSTRACT Coalfield fires have become a major hidden danger in mining operations. In order to analyze the laws of coal oxidation and combustion under stress, a uniaxial pressurized experiment was conducted on briquettes. The results show that as the temperature rises, the oxygen consumption rate of the oxidative combustion of the coal sample increases. This rate is higher in the active stage (120–200°C). CO and CO2 gases begin to be generated in the awakening stage (60–70°C), while hydrocarbon gases are gradually generated in the active stage. Additionally, the index gas escape rate is relatively high under a specific axial pressure (4MP). The cross point temperature of the coal sample at 4MP is lower (86.9°C), and the maximum heating rate is higher (0.117°C/s) compared to other coal-oxygen samples with axial pressure applied. The application of axial pressure has two functions including opening pore channels and compacting particle space, which affect the oxidative spontaneous combustion of coal samples. The research results provide a strong scientific basis and theoretical guidance for the detection and prevention of coalfield fires.

Sustainable utilisation and transformation of the thermal energy from coalfield fires: A comprehensive review

Coalfield fires provide high-temperature heat storage at relatively shallow depths, their overburden and upper strata contain immense amounts of thermal energy, which is identical to the reservoir condition of shallow geothermal energy. Therefore, heat waste resources produced by coalfield fires can be developed and utilised as low-medium grade geothermal resources. By comparing the available methods for thermal energy extraction and conversion, it was found that the heat transfer technology using two-phase closed thermosyphon and thermoelectric conversion using thermoelectric conversion device were the most suitable for use in coalfield fire areas. As a highly effective heat conduction scheme, the two-phase closed thermosyphon was proposed as a feasible means of thermal energy extraction. Meanwhile, thermoelectric conversion device was utilised to transform thermal energy directly into electrical energy to realise the reuse of the waste heat. Some influencing factors on the heat transfer properties of two-phase closed thermosyphon were analysed, and efficiency of the thermoelectric conversion device was investigated. The thermoelectric parameters for these thermoelectric materials of interest were evaluated, and several effective methods for the improvement of thermoelectric conversion efficiency were exposed. In addition, an enhanced thermoelectric conversion and utilisation system with increased conversion efficiency was proposed. The results presented in the current study can provide a theoretical basis and technical support information for the feasible management and adequate control of coalfield fires and utilisation of the resulting thermal energy.

Monitoring Surface Subsidence Using Distributed Scatterer InSAR with an Improved Statistically Homogeneous Pixel Selection Method in Coalfield Fire Zones

Statistically homogeneous pixel (SHP) selection is an important process in the distributed scatterer interferometric synthetic aperture radar (DS-InSAR) approach. However, prevalent methods struggle to appropriately balance the efficiency and accuracy of selection. To solve this problem, the authors of this study improved the Hypothesis Test of Confidence Interval (HTCI) to propose an adaptive method to select the level of saliency and confidence interval for the HTCI, called Adp-HTCI. The proposed method can accurately select homogeneous pixels while inheriting the high efficiency of the HTCI. Once homogeneous pixels have been chosen, the eigenvalue decomposition of the covariance matrix is used to optimize their phase and perform temporal processing. We used the proposed method along with data on 31 scenes from the Sentinel-1 satellite from 2 June 2021 to 28 May 2022 to monitor the deformation of the surface of the fire zone in the Sikeshu coalfield. The selection results of homogeneous pixels indicate that the proposed Adp-HTCI algorithm can increase the number of selected homogeneous pixels while ensuring the accuracy of the selection results, thereby enhancing the estimation accuracy and reliability of subsequent parameter solving. The DS-InSAR results showed that the cumulative maximum subsidence in the study area within a year reached—138 mm and the point density used by the DS-InSAR approach was 17.28 times higher than that used by the StaMPS approach. The results of cross-analysis with the results of StaMPS verified the accuracy of the DS-InSAR-based approach.

Investigation on the quantitative evaluation method of coal combustion situation in O2-CO2-N2 atmospheres based on dynamic artificial neural network

ABSTRACT Due to the semi-enclosed space of underground coal seams and the implementation of nitrogen injection measures, most coalfield fires occur in multi-atmosphere environments. At present, there is little research on the combustion situations and kinetics of coal under multi-atmosphere conditions. In this paper, simultaneous thermal analysis experiments were conducted to investigate the combustion behaviors of bituminous coal and anthracite in O2-CO2-N2 atmospheres with different volume fractions. A new index for quantitative characterization of coal combustion situation in multi-gas mixing atmospheres was proposed based on the combustion situation intensity index and heat release rate. Furthermore, a method to predict the coal combustion situation using artificial neural network (ANN) was proposed. The results showed that the combustion rate decreased with the increase in CO2 volume fraction and the decrease of N2 volume fraction under multi-gas mixing atmospheres. The volume fraction changes in inert gases had little effect on the ignition temperature or burnout temperature and had a greater effect on the combustion reactivity of bituminous coal. In a 15% O2-5% CO2-80% N2 atmosphere, the combustion could be inhibited for bituminous coal, while being promoted by anthracite. The combustion situation of bituminous coal in a 15% O2-25% CO2-60% N2 atmosphere was greater than that in air. Based on the proposed coal combustion situation index and the selected optimal ANN model, the coal combustion situation in O2-CO2-N2 atmospheres could be well predicted. This paper provides a reliable theoretical basis for judging the coal combustion spread in the coalfield fire area.

Study on Flow Field Variation in Fracture Channel of Coalfield Fire

Study on Flow Field Variation in Fracture Channel of Coalfield Fire

Influence of Pore-Crack Environment on Heat Propagation of Underground Coalfield Fire: A Case Study in Daquanhu, Xinjiang, China

Influence of Pore-Crack Environment on Heat Propagation of Underground Coalfield Fire: A Case Study in Daquanhu, Xinjiang, China

Gasification characteristics and thermodynamic analysis of ultra-lean oxygen oxidized lignite residues

Recent developments in the in-situ transformation of coal resources have heightened the possibility of exploring the oxidized coal regasification in coalfield fires. For further understanding the regasification characteristics of oxidized lignite residues in coalfield fire, Shengli lignite was selected for conducting ultra-lean oxygen pre-oxidation under different isothermal temperatures and CO2 regasification experiments. Fourier transform infrared spectroscopy results indicate that higher temperature leads to the increase of aromatic hydrocarbon and reduction of aliphatic hydrocarbon side chains on the coal structure. It is found that the pre-oxidation of under 1% oxygen concentration and 300 °C will increase the specific surface area and pore volume, which would be beneficial for the following regasification. The analysis of gasification parameters suggests that the gasification quality and performance of lignite residues decline with the increasing oxygen concentration and rising isothermal temperatures in pre-oxidation. The kinetic analysis demonstrates that when the oxygen concentration ranges from 1% to 5%, the activation energy of gasification increases by 16.744 kJ/mol. The findings suggest that the lignite pre-oxidation under ultra-lean oxygen conditions will inevitably increase the activation energy of CO2 gasification, which causes the coal more difficult to be utilized effectively.

Pyrolysis Characteristics of Jet Coal and Oxidation of Residues in Zhundong Coalfield Fires.

Coalfield fire area reburning is one of the serious disasters in fire prevention and safety production. In this study, a synchronous thermal analyzer was used to conduct isothermal pyrolysis of jet coal at different temperatures, and the reaction characteristic parameters of different pyrolysis residual structures were analyzed. FTIR was used to measure group contents in raw coal and different pyrolysis residues. Programmed oxidation thermogravimetric experiments were carried out on the residues to obtain their oxidation characteristic parameters. The results demonstrated that the reaction characteristic parameters of the residual structures changed at 450 °C. The pyrolysis reaction mainly affected the variation of hydroxyl, aliphatic hydrocarbon, and aromatic hydrocarbon groups. The increase in pyrolysis temperature resulted in the decline in hydroxyl and aliphatic hydrocarbon groups as well as the increase in the aromatic hydrocarbon group. After pyrolysis, the ignition point temperature of the coal sample decreased, which causes the coal more likely to spontaneously ignite. It indicated that the pyrolysis residue at 450 °C is most likely to reburn. Compared with raw coal, the maximum combustion intensity of the pyrolysis residue was greatly increased, which reached the peak at 500 °C.

The structural transformation and reburning characteristics of gas coal in Ningwu coalfield fire

With the expansion of the scale of coal mining, the safety problems caused by the reburning of coal are becoming more and more serious. In this paper, the pyrolysis characteristics of gas coal and the exothermic characteristics of reoxidation of residues were studied by using a synchronous thermal analyzer. The functional groups of pyrolysis residues were tested, and the group content and characteristic structural parameters were calculated based on quantum chemistry method. The results show that with the increase of pyrolysis temperature, Volatile maximum separation rate (Vmax) and the change in the residual weight of the coal sample (ΔWvp) increase. The increase of temperature will lead to the decrease of hydroxyl and aliphatic hydrocarbon content in coal, and the increase of aromatic hydrocarbon. With the deepening of pyrolysis, the ignition point temperature of coal samples decreases first and then rises, the combustion intensity and combustion concentration are strengthened. The pyrolysis results show that 462.8°C is the critical temperature for the transition during pyrolysis. The ignition point of the residue is less affected by the pyrolysis conditions, and the ignition temperature of the raw coal and the pyrolysis residue varies within 330.57°C–334.98°C.

Changes in Microcrystalline Composition, Functional Groups and Combustion Characteristics of Coal in Coalfield Fire Area after Baking by Different Temperature Gradients

ABSTRACT Once fires occur in a coalfield, large quantities of coal in the coalfield will be baked at high temperatures. The changes and differences in the microscopic and spontaneous combustion properties of coals in high-temperature environments (ambient temperature or even after high-temperature baking) in the fire zone have not been studied yet. In this paper, three types of different-rank coals were baked at five different temperatures to investigate their changes in microcrystalline compositions, functional groups and combustion characteristics. The following results were obtained. The temperature treatment causes damage to the microcrystalline structure of different-rank coals. As the baking temperature rises, the aromatic layer spacing decreases, while the ductility and stacking height gradually increase. The aromatic ether content and the aliphatic hydrocarbon content gradually decline, while the -COOH content goes first and then falls. The spontaneous combustion activities of coals around the fire area gradually weaken with the rise of baking temperature. This research is crucial for revealing the effect of temperature, especially fire baking, on the spontaneous combustion characteristics of coal.

Underground coal fire emission of spontaneous combustion, Sandaoba coalfield in Xinjiang, China: Investigation and analysis.

Long-term spontaneous combustion of coal has caused serious ecological and environmental problems. Only in recent years has it received growing popularity to undertake relevant researches. In order to study the impact of combustion by-products on atmosphere in the Sandaoba fire field, Xinjiang, a region-scale field survey was firstly conducted to investigate the gaseous-solid emissions in separated fire sections. The evaluation method and model have been proposed to describe the underground combustion and the related air pollution. Every year, the total estimates of the gaseous emission are approximately 4030t of CO2, 113.6t of SO2 and 57.3t of CO. The emission pollution varies considerably from regions, and is substantially attenuated with the advancement of fire control. Principal component analysis (PCA) refines the thermophysical parameters into three attributions: the intrinsic thermophysical property, atmospheric dynamics, and combustion degree. PCA score distribution shows that thermophysical parameter is dominated by the combustion condition at severely polluted areas. Factor Analysis is used to extract four contaminant indicators, which suggests the local air suffers sulfur oxides pollution the most. The air quality index of the eight study sections calculated are all below 60, ranging from 24 to 58. It indicates that coal fire air pollution is in the medium-to-severe stage. By Canonical Correlation Analysis, it is noted that thermophysical indicator performs outstanding explanatory for contaminant variates. On the whole, the higher the level of thermophysical properties in the fire area, the greater the intensity of pollutant emission. Underground coalfield fire is dominated by smoldering, and the overall combustion efficiency is lower than 0.8 which generally declines as the excess air coefficient increasing.

Detecting concealed fire sources in coalfield fires: An application study

A majority of coal mines in China are susceptible to fires, especially those affected by concealed fire sources in coalfield fire areas. Hence, to mitigate this problem, the efficient and precise detection of high-temperature anomalous areas is significant and needed, which mainly involves determining the range and depth of concealed fire sources. In this paper, the strengths and weaknesses of each detection method (e.g., remote sensing, electromagnetic radiation, magnetic detection, geological radar detection, resistivity detection, transient electromagnetic, excitation potential method, gas analysis, radon levels detection, infrared images) were investigated, analyzed, evaluated and then compared. Furthermore, after taking their respective practicality and effectiveness into account, an integrated methodology was proposed that uses radon levels, infrared imaging techniques, and drilling methods to detect and verify high-temperature anomalous areas in a coalfield. We carried out the detection in the Ningxia autonomous region. Our testing results revealed that radon levels show positive correspondence with the temperature of potential fire sources. That is to say, higher radon levels usually have higher temperature signal for detection. Accordingly, high-temperature anomalous areas in coalfield fire areas can be distinguished and delineated successfully by their radon levels and verified by infrared imaging techniques and drilling methods.