Underground Coal Fires Research Articles

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.

Specification of complex-PAHs in coal fire sponges (CFS) by high-resolution mass spectrometry with electrospray ionization.

Underground coal fires are considered an ecological disaster. While underground coal fires are prevalent in coal-producing areas throughout the world, they are most problematic in northern China. Previous studies have shown that underground coal fires stimulate the formation of cracks or gas outlets on the surface, as well as coal fire sponges (CFS) on the soil layer surface, which collect coal-fired pollutants. Herein, ultra-high-performance liquid chromatography (UHPLC) was used in conjunction with electrospray ionization (ESI) high-resolution mass spectrometry to analyze CFS samples collected from the No. 8 fire zone, located in Wuda coalfield, Inner Mongolia, China. The results show that CFS contain 233 oxy-substituted polycyclic aromatic hydrocarbons (O-PAHs), e.g., naphthaldehyde; 40 oxapolycyclic aromatic hydrocarbons (OPAHs), e.g., dibenzofuran; 40 alkyl-substituted polycyclic aromatic hydrocarbons (R-PAHs); and 11 parent polycyclic aromatic hydrocarbons (PPAHs). Thus, CFS are primarily composed of O-PAHs, which are 25 times and 5 times more prevalent than PPAHs and R-PAHs, respectively. As such, a high relative abundance of varied O-PAHs are discharged from underground coal fires, which is significantly different from what is released during industrial coal burning. Owing to their water solubility and condensability, the new facts disclosed in this paper may provide a new perspective for understanding complex organic pollutants from underground coal fires and their environmental impacts.

Numerical stability analysis of superhigh-fill subgrade underlying acute inclined mined-out area with underground coal fire

According to the borehole drilling detection results of a superhigh-fill highway subgrade that crossed an open-pit coal mined-out area with an underground coal fire (UCF) in Urumqi, China, a numerical model of the subgrade and substrata was established. The variations of the stability characteristics of the highway pavement and the subgrade slope with respect to the substrata properties under the effects of coal-seam combustion were analyzed via thermal-stress coupled calculations using the finite-difference software FLAC3D. When the coal body underlying the subgrade underwent burnout and the combustion cavity formed, the bearing capacity of the strata decreased significantly. Under the effect of the heavy self-weight of the high-fill subgrade body and other external loads, such as the vehicle load on the pavement, collapse was likely, followed by the formation of a large sink basin on the surface of the subgrade. The displacement of the highway suffering from UCF increased gradually. The displacement development was divided into two stages: the swelling stage due to rock-mass expansion and the collapse stage due to the burnout of the coal body. The concrete geocell had a good reinforcement effect on the subgrade in which the settlement of the basin occurred under the effects of the UCF. When part of the geocell was deformed under external loads, the loads were transferred to the neighboring concrete geocell located in the stable region. Thus, the concrete geocell in the stable region constrained the geocell in the unstable region from deforming. Furthermore, a parametric analysis with varying combustion conditions indicated that a higher heat-release intensity, wider combustion region, and shallower combustion depth lead to greater stability hazards for the overlying subgrade.

Study on the electromagnetic spectrum characteristics of underground coal fire hazardous and the detection criteria of high temperature anomaly area

Electromagnetic radiation (EMR) is an optimal non-contact and directional geophysical detection method, which has been applied in geotechnical engineering, monitoring and giving early warning about the coal and rock dynamic disaster. In this paper, the different frequencies EMR signals generated in the process of coal heating and spontaneous combustion are tested and analyzed, and the detection criterion of locating anomalous heating areas by EMR is illustrated. During the spontaneously-heating process of coal, the EMR signals are detected by antennas with frequencies of 30 kHz, 100 kHz, 300 kHz and 1 MHz, covering both low and high frequencies. In the process of coal spontaneous combustion, the spectrum of EMR signal changes with the increase of coal temperature. The amplitude of EMR also fluctuates, showing the characteristics of high frequency and high amplitude, and the corresponding thermal damage of coal occurs in different degrees (intensities). Further, in order to use EMR as an effective detection method of coal fire, the relationship between the intensity of EMR and coal size, test distance and temperature is discussed. With the dominant spectrum of EMR determined by field tests, researchers can employ different EMR frequency to detect the fire risk based on the concrete conditions of coal field. This study provides a meaningful method for improving the level of safety and environmental protection during mining process.

Spatial pattern of land surface temperatures and its relation to underground coal fires in the Khanh Hoa Coal Field, North-East of Vietnam

Change of land surface temperatures (LSTs) is indicative of underground coal fires (UCFs). The limitation of commonly used methods for UCF detection is that spatial pattern of LSTs is not taken into account. This study aims to identify spatial pattern of LSTs retrieved from remotely sensed data and its relation to UCFs. LSTs were firstly retrieved from the Landsat-8 TIRS data using the radiative transfer equation (RTE). The local Moran’s I statistics was then used to identify the spatial pattern of these LSTs. Different degrees of spatial pattern (low, medium, high, extreme high, lower, and upper outliers) were then identified by setting the first, second, third, and fourth quartiles and the MEDIAN + 1.5 × IQR (IQR is the interquartile range) and MEDIAN + 3 × IQR formulas as thresholds. The relation of LST spatial pattern to UCFs was finally identified by overlapping spatial pattern layers on known active UCF sites and compared with those reported previously. Results obtained from 2nd December 2013 Landsat-8 TIRS data in the Khanh Hoa coal field (north-east of Vietnam) showed that LSTs followed a pattern of spatial clustering of high values in the coal field. The areas with the degrees of spatial correlation in the quartiles of 0–25%, 25–50%, 50–75%, and above 75% were 66.5, 66.9, 34.1, and 71.1 ha, respectively. Lower and upper outliers were detected at positions of 12.38 and 21.31 corresponding to the areas of 23.4 and 4.6 ha, respectively. These outliers were mainly concentrated around eight active UCF sites and were highly consistent with those obtained from previous studies. The results of this investigation show that the closer the UCF sites, the higher the spatial autocorrelation level. There exists a strong degree of positive correlation between the distribution of LST spatial pattern with active UCFs. These findings suggest that high degree of autocorrelation of LSTs can be used to effectively detect UCFs.

Effects of Elevated Temperatures on the Microstructural, Physico-Mechanical and Elastic Properties of Barakar Sandstone: A Study from One of the World’s Largest Underground Coalmine Fire Region, Jharia, India

Experimental investigations of the effects of elevated temperature on microstructural, mineralogical, physico-mechanical and elastic properties of rocks were carried out to understand the effect of underground coal fire along with thermal damage and failure initiation in Barakar sandstone from the Jharia coalfield, India. Discolouration analysis, petrographic studies, scanning electron microscopy, thermogravimetry, differential thermal analysis and electron probe micro analysis were carried out to understand the microstructural and mineralogical changes in thermally treated samples. The experimental results suggest that the changes in investigated properties follow three characteristic thermal zones: zone I (25–300 °C), zone II (300–500 °C) and zone III (500–800 °C). Among all, zone II is a prominent transition region where a rapid increase in crack density, porosity and thermal damage is observed along with a sharp reduction in seismic wave velocities, uniaxial compressive strength, tensile strength and elastic parameters. Development of thermal stress-induced intergranular and intragranular micro-cracks, evaporation of organic matters, and dehydration and thermal decomposition of minerals are the most conspicuous reasons for the observed changes in this zone. Zone III is characterized by phase transformation of quartz, recrystallization and partial melting of cement and phyllosilicates that filled some of the existing micro-cracks, resulting in a local change of porosity, strength and elastic properties. Developments of different failure modes are observed in different temperature ranges such as simple shear along a single plane in zone I, progressive growth of Y-shaped (double plane) shearing in the zones II and III (400–700 °C) and axial splitting at 800 °C. The results from this study provide comprehensive elaboration of thermal effects on sandstones at a particular temperature range.

Monitoring coal seam fires in Xinjiang using comprehensive thermal infrared and time series InSAR detection

ABSTRACT The combustion of underground coal seams has important effects on the national economy and strategic energy security of countries, and damages the environment, ecology, vegetation, and water resources. Xinjiang, China, is one of the most serious areas of coalfield fires in the world, and it is difficult for any single remote sensing detection method to meet the practical requirements for accurate detection of fire areas. In order to detect coalfield areas in Xinjiang more accurately and reliably, a detection method that integrates thermal infrared and radar remote sensing anomaly is proposed: The mono-window algorithm is used to retrieve the Land Surface Temperature (LST) using images from the Land Remote-Sensing Satellite System (Landsat) 8, the Adaptive Edge Threshold Algorithm (AETA) is used to extract the data characterized by LST anomalies, and Small Baseline Subsets Interferometric Synthetic Aperture Radar (SBAS-InSAR) is used to obtain the land surface subsidence information from Sentinel-1A images. Through the overlay analysis of the above-mentioned LST and subsidence information, the extraction of possible coal fire areas in the Jiangjun Gobi area of Xinjiang is realized. As well as according to the characteristic that underground coal fires melt snow in winter, areas where coal fires are unlikely to occur are excluded using the natural, true-colour images of Landsat 8. Finally, the proposed method is verified using the characteristics of the time series cumulative subsidence of the land surface in areas where coal fires are likely to occur, and by examining the coupling between subsidence velocity and anomalies in LST data. The results show that the proposed method can overcome the shortcomings of single-factor remote sensing detection to a certain extent, and has the ability to detect fire areas more accurately.

Thermogravimetric and infrared spectral analysis of candle coal pyrolysis under low-oxygen concentration

For further understanding spontaneous combustion of coal in fire suspectable zone, this paper analyzed coal pyrolysis in low-oxygen conditions by thermogravimetric analysis and online FTIR. The results indicate that the oxidation and pyrolyzation of candle coal occur simultaneously at 3 %∼5 % oxygen concentration, the intensity of oxidation reaction can still promote the coal self-heating. With the decrease of oxygen concentration and temperature, the CC on benzene ring decreases while the oxygen-containing groups increase. With oxygen concentration decreasing continuously, the oxygen-containing groups begin to appear surplus. When the temperature rises to 500 °C, the content of −COOH and −OH still remains 0.3 % under 3 % oxygen concentration, but disappears at 5 % oxygen concentration. It indicates that 3 % oxygen supply cannot provide necessary oxidation conditions for candle coal. Consequently, in order to fully suppress the development and re-burning of underground coal fire, the oxygen concentration should be controlled below 3 %.

Temperature characteristic of crushed coal under liquid coolant injection: a comparative investigation between CO2 and N2

For large-scale underground coal fires (LUCF), how to achieve significant cooling was a key to prevent and extinguish spontaneous combustion of coal in mining-out area. The liquid CO2 or liquid N2 injection, which has stronger cooling, wider coverage, and inerting effect, was a reliable technology that can improve the efficiency of LUCF control in the coal industry. However, before application, it is of fundamental importance for understanding of the heat transfer mechanisms and temperature variations of crushed coal during and after liquid CO2/N2 injection. In this paper, a series of laboratory experiments was designed and used to research the process about the phase change and convective diffusion of liquid CO2 and liquid N2 in coal under controlled conditions. Liquid CO2/N2 was separately injected into a cylindrical container which is filled with crushed coal, and the transient variation of coal temperature was measured on specified sites in the system by using temperature sensors. A comparative analysis between the liquid CO2 and liquid N2 injection for temperature characteristic of coal was conducted. In addition, the temporal-spatial characteristics of the cooling energy caused by latent heat of phase transformation of liquid CO2 and liquid N2 were revealed, and its correlation with the transient coal temperature field was identified.

Modeling of dispersion behaviors during the injection of liquid carbon dioxide in pulverized coal with experimental validation

This article presents a model for investigating the thermo- and fluid-dynamical behaviors of liquid carbon dioxide (LCO2) in pulverized coal, which is coupling of phase transition, multiphase flow, heat and mass transfer. In the model, the intensive non-isothermal effect caused by phase transition during the injection process is considered as a negative heat source associated with transient latent heat and the temperature–dependent physical parameters based on Span–Wagner equations are employed. By comparison with the data from model predictions and test under various nozzle diameters, the proposed model has been validated against experimental results from temperature variation. In addition, the dispersion behaviors of LCO2 in pulverized coal are extensively studied. The presented research provides a viable method for the prediction of dispersion and thermal behaviors when LCO2 is used for underground coal fires prevention and mitigation.

Gas Emission and Soil Chemical Properties Associated with Underground Coal Fires, Wuda Coalfield, Inner Mongolia, China

The coal fire area in the Wuda coalfield is divided into four parts based on the degree of burning and on surface characteristics: sub-area B is characterized by degraded vegetation, sub-area A has spontaneous combustion but no obvious features, sub-area O has vents and cracks, and sub-area C is characterized by red pyro-metamorphic rocks. Emitted gas contents and temperature at the surface were measured and analyzed. In addition, soil samples were collected for analysis of chemical soil properties. The compositions and concentrations of emitted gases vary from one sub-area to another. The nearly the same correlations among gas contents in sub-area A and in sub-area O indicate that their generation mechanism is the same. SO2, H2S, C2H6, C2H4, and C2H2 can only be detected near the cracks and vents in sub-area O. CO and H2 can still be detected in sub-area C, although the CO2 and CH4 contents in sub-area B are higher. Sub-areas A and B exhibit high organic matter contents due to plant nutrient accumulation and deposition of plant litter, whereas organic matter contents decrease at the vents and cracks in sub-area O and in sub-area C because of pyrolysis, leaching, and erosion. Soil near the vents and cracks in sub-area O show strong acidity, and soil pH is correlated with total salt contents of soil, which are composed mainly of Al-, K-, Mg-, or Fe-bearing sulfates. Sub-area A showed weak soil acidity, which was the result of acidification of pore water in soil due to their long-term exposure to CO2 and SO2. Total salt contents in soil in the coal fire area show no significant difference compared to those in the background area except near the vents and cracks in sub-area O. The findings of this study suggest that gas emission and soil chemical properties are reliably related to the underground coal fires.

Underground Coal fire Mapping Using Analysis of Self-potential (SP) Data Collected from Akashkinaree Colliery, Jharia Coalfield, India

Abstract Sub-surface coal mine fires burn valuable resources and give rise to great threat in the local environs and people’s health. Jharia coal field is one of utmost fire affected coal fields in India. This paper deals with the study of sub-surface coal fire using self-potential (SP) method, a cost-effective and non-invasive passive geophysical technique. SP method in coal fire mapping works based on redox potential generated by oxidation of coal and also on Thomson potential due to temperature gradient. Initially, SP anomaly map generated using electrode drift corrected field data. The SP anomalies in the present study area vary from ~95 to ~ -40mV. SP anomaly map has been continued to a height of 10m from the ground to remove the possible shallow surface noises. The upward continued anomaly map has been enhanced using first vertical derivative. Further, Euler’s depth solutions have been estimated to understand possible geometry and source depth of sub-surface coal fires. The local geological map and field photograph corroborate well with the results for the characterization of coal seam fires.

Falgarite, K4(VO)3(SO4)5, a new mineral from sublimates of a natural underground coal fire at the tract of Kukhi-Malik, Fan-Yagnob coal deposit, Tajikistan

AbstractA new mineral falgarite, K4(V+4O)3(SO4)5 was discovered at the tract of Kukhi-Malik, Fan-Yagnob coal deposit, ca. 75 km N of Dushanbe, Tajikistan. The new mineral is named after the Falgar, an ancient Sogdian name for an area around the Zeravshan riverhead. Falgarite is a fumarolic mineral formed directly from a gas emitted by a natural underground coal fire. Associated minerals are anhydrite, baryte, molybdite, an unidentified Tl-vanadyl sulfate, K–Mg sulfate and an anhydrous Mg-sulfate. Falgarite forms small isometric or pseudo-octahedral individual crystals (10–60 μm) of turquoise colour and spherical aggregates up to 0.5 mm in diameter. Mohs hardness is ~ 2.5, Dmeas = 2.87(2) and Dcalc = 2.89 g/cm3. Refractive indices are: α = 1.588(3), β(calc.) = 1.600(3) and γ = 1.609(2) (590 nm). In transmitted light falgarite is transparent green with a weak pleochroism. The mineral is non-soluble in H2O and 5% HNO3 at room temperature. Infrared spectra support the absence of H2O and OH–. The chemical composition determined by electron-microprobe analysis is (wt.%): Na2O 0.55, K2O 20.76, Tl2O 1.83, VO2 29.38 and SO3 46.78, total 99.29. The empirical formula (based on 23 O apfu) is: (K3.76Na0.15Tl0.07)Σ3.98V3.02S4.99O23.0. The strongest lines of the powder X-ray diffraction pattern are [d,Å(I,%)(hkl)]: 3.20(70)(202); 3.17(80)024; 3.14(70)$\bar{2}$04; 3.01(50)$\bar{1}$51; and 2.88(100)151. Falgarite is monoclinic, P21/n, a = 8.7209(5), b = 16.1777(6), c = 14.4614(7) Å, β = 106.744(5)°, V = 1953.77(17) Å3, Z = 4 and R1 = 0.05. VO6 octahedra and SO4 tetrahedra link together by sharing corners thus forming a [(VO)3(SO4)5]4– framework. K+, Na+ and Tl+ cations are located in the channels of the framework. The synthetic K4(VO)3(SO4)5 analogue is known.

Chimney effect induced by smoldering fire in a U-shaped porous channel: A governing mechanism of the persistent underground coal fires

This paper presents underground coal fires (UCF) induced natural ventilation through a U-shaped porous channel. Height of the U-shaped channel (the fire depth) is one of key elements determining the accessibility of air supply to UCF. Conventionally, we acknowledge that under the external wind driving force, air supply to underground space should decay with increasing the fire depth. However, under the thermal buoyancy force induced by UCF, responses of air supply and UCF to the fire depth are uncertain. Herein we propose a 1/20-scale experimental framework to measure air velocity, and to quantify the burning rate, the fire spread rate and the burning temperature of UCF at different fire depths (H = 1.6–4.6 m) with variable aperture sizes (Φ = 1–4 cm). A one-dimensional model correlating the air velocity with the fire depth is validated and then extrapolates laboratory-scale free channels into field-scale (H = 100 m) percolation channels. We find the ‘chimney effect’ – air supply driven by the buoyant smoke of UCF is unexpectedly enhanced with increasing the fire depth; the enhanced air supply due to the chimney effect facilitates burning of coal. The chimney effect, serving as a self-sustaining mechanism of air supply to UCF, is a significant governing mechanism for persistent UCF burning for hundreds or even thousands of years.

Experimental investigation of the impact of liquid carbon dioxide injection on temperature in pulverized coal

AbstractUsing liquid carbon dioxide (LCO2) as a medium for underground coal fires prevention and control, while reducing the emissions of CO2, is a promising option for CO2 utilization worldwide. However, the mechanism of LCO2 flow with phase transition on coal temperature is still not clear. In this work, an experimental system was designed and fabricated to investigate the phase transition behavior and temperature variation during injecting LCO2 into pulverized coal and the impact of mass flow rate, nozzle diameter, and injection pressure on cooling effect. The results indicate that when LCO2 was injected into the pulverized coal, the instantaneous phase change into solid and vapor occurred just after leaving the release port. An intensive heat transfer process occurred because of the throttling effect, flashing effect, and sublimation and thereby formed a cooling area (<−56.6°C), which was the main area of cooling effect and phase transition; as injection time increases, the range of this area increases in the form of logarithmic function. In addition, the cooling area exhibited a strong dependence on the injection conditions, and its correlation with mass flow rate, nozzle diameter, and injection pressure are identified as exponential, logarithmic, and linear relationship by dimensionless analysis, respectively. Meanwhile, an empirical formula was developed for calculating the cooling effect under various injection conditions. In summary, the experiments provided fundamental data and regime for predicting the effect of LCO2 injection on the temperature of pulverized coal, which can be used to guide the utilization of LCO2 for coal fires prevention. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

Study on inversion of coal seam temperature in mining area --Pingshuo mining area of Shanxi Province

Landsat 8 is widely used in the extraction of surface temperature, but the data of surface temperature and abnormal area in Pingshuo mining area is vacant based on Landsat 8 in recent years, and there is no standard optimal algorithm to follow. In order to explore the possibility of underground coal fire in Pingshuo mining area of Shanxi Province in the future, based on the Landsat 8 satellite data, the temperature inversion method is used to observe the temperature distribution of the mining area, and three commonly used algorithms of temperature inversion processing are used to compare and analyze the SC algorithm as the best data processing method. The artificial threshold method and NDVI threshold method are used to extract the temperature anomaly area and vegetation coverage area, and calculate the area and proportion of coal fire potential area. According to a series of the data and result charts analysis, it shows that: the highest vegetation index of Pingshuo mining area is 0.79, the vegetation coverage is low, and the surface temperature is more than 41.44 ℃, which may lead to the spontaneous combustion of underground coal mines. However, the area prone to underground coal fires is small and controllable. According to the area of potential coal fires in the mining area, the local relevant departments can take relevant measures to prevent coal fire through the distribution map of potential coal fires.

An Improved Composite Fly Ash Gel to Extinguish Underground Coal Fire in Close Distance Coal Seams: A Case Study

Fire extinguishing with the superior performance of fly ash composite colloid material is a cost‐effective colloid fire prevention technology. In this paper, a new powder additive (suspending agent (XK‐XJ) and gelling agent) was developed for the existing fire extinguishing technology of fly ash compound colloid. Tests show that the best additions to the fly ash slurry were 0.3% and 0.1∼0.3%, respectively. The grouting technical scheme of adding a suspending agent on the ground and gelling agent downhole was proposed to solve the problems of solid material settlement and blockage in the long‐distance pipeline transportation process. Finally, the optimized fly ash colloid fire prevention and extinguishing technology was successfully applied to the fire control engineering examples of close‐range coal seam groups, and the rationality of the developed powder and its proportion was tested, and the feasibility in coal fire control was tested.

A laboratory approach to CO2 and CO emission factors from underground coal fires

Carbon emissions from underground coal fires (UCF) have become an emerging research topic and their role in global climate warming has been widely debated. Currently, one big uncertainty for assessing UCF's carbon emission is the hypothesized carbon emission factors (EF) from the complete combustion of coal, while the EF of smoldering combustion of coal in the context of UCF is still unknown yet. In this work, a 1/20 scale laboratory experimental framework was proposed to characterize transient carbon emissions and quantify EFCO2 and EFCO. Effects of fire depth, ventilation area (aperture size), and coal rank on carbon emissions were explored with the extrapolation to the full-scale UCF. Results showed that total carbon emissions increase with the carbon content of coal. Volatile content is an important factor impacting the burning behavior and gas emission. Stable EFCO2 and EFCO of UCF, independent of the fire depth and aperture size, were estimated as 2006 ± 36 g kg−1 and 345 ± 132 g kg−1, respectively; its combustion efficiency was 85% ± 3%. The extrapolation of experimental data estimates the CO2 emission of coal fires in China and the USA as 2.34 × 107–4.61 × 107 t yr−1, which accounted for 0.4% - 0.9% of total CO2 emissions in the world in 2016.

On the effects of temperature-dependent diffusion of carbon dioxide from underground coal fires

This study offers a physics-based model for the assessment of the CO2 emission through the overburden above underground coal fires. It was developed by coupling simultaneous porous media subsurface flow, heat transfer, and solute transport phenomena to create an accurate underground coal fire/overburden model. Parametric studies were conducted by varying coal fire and overburden temperatures, soil type and void ratios. The results show that as the overburden and the coal fire temperatures increased, a lower CO2 concentration at the top surface of the overburden was observed due to increased CO2 storage in the overburden and reduced temperature-dependent diffusivity.

Sources and Assessment of Mercury and Other Heavy Metal Contamination in Soils Surrounding the Wuda Underground Coal Fire Area, Inner Mongolia, China.

Concentrations of mercury and other heavy metals were measured in soils surrounding the Wuda Coalfield, Inner Mongolia where underground coal fires are regarded as an important contamination source. Soil samples at the downwind direction were nearly all enriched mercury in surface layers, with their concentrations ~ 10 times higher than those from upwind soils. Mercury concentrations generally increased along soil profiles upward, indicative of increasing atmospheric mercury deposition in recent decades. We suggest that underground coal fire emissions were the potential mercury contamination source and the key factor controlling mercury enrichment in soils. Other heavy metals showed no clear spatial and vertical trends, suggesting that underground coal fire emissions may not the main factor for their enrichment in soils. Antimony showed the highest enrichment followed by tin, while chromium, mercury and cadmium only exhibited slight enrichment. Risk evaluation indicated that heavy metals in the studied soils pose no ecological threats.