Disasters of gas-coal spontaneous combustion in goaf of steeply inclined extra-thick coal seams

In light of the escalating global energy imperatives,mining of challenging-to-access resources,such as steeply inclined extra-thick coal seams(SIEC),has emerged as one of the future trends within the domain of energy advancement.However,there is a risk of gas and coal spontaneous combustion coupling disasters(GCC)within the goaf of SIEC due to the complex goaf structure engendered by the unique mining methodologies of SIEC.To ensure that SIEC is mined safely and efficiently,this study conducts research on the GCC within the goaf of SIEC using field observation,theoretical analysis,and numerical modeling.The results demonstrate that the dip angle,the structural dimensions in terms of width-to-length ratio,and compressive strength of the overlying rock are the key factors contributing to the goaf instability of SIEC.The gangue was asymmetrically filled,primarily accumulating within the central and lower portions of the goaf,and the filling height increased proportionally with the advancing caving height,the expansion coefficient,and the thickness of the surrounding rock formation.The GCC occurs in the goaf of SIEC,with an air-return side range of 41 m and an air-intake side range of 14 m,at the intersection area of the“<”-shaped oxygen concentration distribution(coal spontaneous combustion)and the“>”-shaped gas concentration distribution(gas explosion).The optimal nitrogen flow rate is 1000 m3/h with an injection port situated 25 m away from the working face for the highest nitrogen diffusion efficacy and lowest risk of gas explosion,coal spontaneous combustion,and GCC.It has significant engineering applications for ensuring the safe mining of SIEC threatened by the GCC.

Effect and Mechanism of Rare Earth Hydrotalcite Inhibiting Coal Spontaneous Combustion

A hydrotalcite(layered double hydroxide, LDH) inhibitor which is suitable for the whole process of coal spontaneous combustion and a LDH inhibitor containing rare earth lanthanum elements were prepared. The inhibition effect and mechanism were analyzed by scanning electron microscopy(SEM),X-ray diffraction(XRD), thermal performance analysis, in-situ diffuse reflectance infrared spectroscopy and temperature-programmed experiment. The results have shown that the inhibitor containing lanthanum can play a good inhibitory role in every stage of coal oxidation. During the slow oxidation of coal samples, the inhibitor containing lanthanum ions can slow down the oxidation process of coal and increase the initial temperature of coal spontaneous combustion. At the same time, because the hydroxyl groups in LDHs are connected with-COO-groups on the coal surface through hydrogen bonds, the stability of coal is improved. With the increase of temperature, LDHs can remove interlayer water molecules and reduce the surface temperature of coal. CO release rate of coal samples decreases significantly after adding inhibitor containing lanthanum element, and the maximum inhibition rate of the inhibitor is 58.1%.

Method for prevention and control of spontaneous combustion of coal seam and its application in mining field

Spontaneous combustion of coal seam has been and continues to be a big problem in coal mines. It could pose great threat to the safety of the whole mine and all miners, especially when it occurs in or nearby coal mines. Besides, environment of area surrounded mines during combustion can be threatened where large amount of toxic gases including CO_2, CO, SO_2 and H_2S can be leased by fire in mine. Hence, it is important and significant for scholars to study the controlling and preventing of the coal seam fire. In this paper, the complicated reasons for the occurrence and development of spontaneous combustion in coal seam are analysed and different models under various air leakage situations are built as well. Based on the model and approximately calculation, the difficulty of fire extinguishment in coal seam is pointed out as the difficulty and poor effect to remove the large amount of heat released. Detailed measurements about backfilling and case analyses are also provided on the basis of the recent ten years' practice of controlling spontaneous combustion in coal seams in China. A technical fire prevention and control method has been concluded as five steps including detection, prevention, sealing, injection and pressure adjustment. However, various backfill materials require different application and environmental factors, so in this paper, analyses and discussion about the effect and engineering application of prevention of spontaneous combustion are provided according to different backfilling technologies and methods. Once the aforementioned fire prevention can be widely applied and regulated in mines, green mining will be achievable concerning mine fire prevention and control.

Ambient nanoparticles/nanominerals and hazardous elements from coal combustion activity:Implications on energy challenges and health hazards

Coal is the most abundant fossil fuel in the world. Because of the growth of coal mining, coal-fired power plants and coal-burning industries, the increase of the emission of particulates(coarse, fine or ultrafine)is of great concern. There is a relationship between increasing human morbidity and mortality and progressive environmental air pollution caused by these types of particles. Thus, the knowledge of the physico-chemical composition and ambient concentrations of coal-derived nanoparticles will improve pollution control strategy. Given the current importance of this area of research, the advanced characterization of this coal combustion-derived nanoparticles/nanominerals as well as hazardous elements is likely to be one of the hottest research fields in coming days. In this review, we try to compile the existing knowledge on coal-derived nanoparticles/nanominerals and discuss the advanced level of characterization techniques for future research. This review also provides some of aspects of health risks associated with exposure to ambient nanoparticles. In addition, the presence of some of the hazardous elements in coal and coal combustion activities is also reviewed.

Transformation of alkali and alkaline-earth metals during coal oxy-fuel combustion in the presence of SO_2 and H_2O

The occurrence modes of alkali and alkaline-earth metals（AAEMs） in coal relate to their release behavior and ash formation during combustion. To better understand the transformation of AAEMs,the release behavior of water-soluble,HCl-soluble,HCl-insoluble AAEMs during Shenmu coal（SM coal） oxy-fuel combustion in the presence of SO2 and H2O in a drop-tube reactor was investigated through serial dissolution using H2O and HCl solutions. The results show that the release rates of AAEMs increase with an increase in temperature under the three atmospheres studied. The high release rates of Mg and Ca from SM coal are dependent on the high content of soluble Mg and Ca in SM coal. SO2 inhibits the release rates of AAEMs,while H2O promotes them. The effects of SO2 and H2O on the Na and K species are more evident than those on Mg and Ca species. All three types of AAEMs in coal can volatilize in the gas phase during coal combustion. The W-type AAEMs release excessively,whereas the release rates of I-type AAEMs are relatively lower. Different types of AAEM may interconvert through different pathways under certain conditions. Both SO2 and H2O promote the transformation reactions. The effect of SO2 was related to sulfate formation and the promotion by H2O occurs because of a decrease in the melting point of the solid as well as the reaction of H2O.

The Cellular Toxicity of PM_(2.5) Emitted from Coal Combustion in Human Umbilical Vein Endothelial Cells

Objective To explore the relationship between different components of fine particulate matter（PM2.5） emitted from coal combustion and their cytotoxic effect in the vascular endothelial cells. Methods Coal-fired PM2.5 was sampled using a fixed-source dilution channel and flow sampler. The sample components were analyzed by ion chromatography and inductively coupled plasma atomic emission spectroscopy（ICP-AES）. The PM2.5 suspension was extracted using an ultrasonic water-bath method and then human umbilical vein endothelial cells（EA.hy926） were treated with various concentrations of the PM2.5 suspension. Cell proliferation, oxidative DNA damage, and global DNA methylation levels were used to measure the cellular toxicity of PM2.5 emitted from coal combustion. Results Compared to other types of coal-fired PM2.5 preparations, the PM2.5 suspension from Yinchuan coal had the highest cytotoxicity. PM2.5 suspension from Datong coal had the highest toxic effect while that from Yinchuan coal had the lowest. Exposure to coal-fired PM2.5 from Jingxi coal resulted in lower 8-hydroxy-2’-deoxyguanosine（8-OHd G） levels. At the same dose, PM2.5 emitted from coal combustion could produce more severe DNA impairment compared to that produced by carbon black. Cell survival rate was negatively correlated with chloride and potassium ions content. The 5-methylcytosine（5-m C） level was positively correlated with Mn and negatively correlated with Zn levels. The 8-OHd G% level was positively correlated with both Mn and Fe. Conclusion PM2.5 emitted from coal combustion can decrease cell viability, increase global DNA methylation, and cause oxidative DNA damage in EA.hy926 cells. Metal components may be important factors that influence cellular toxicity.

Effects of pyrite on the spontaneous combustion of coal

Abstract Pyrite has a significant effect on the spontaneous combustion of coal. The presence of pyrite can change the propensity of coal towards spontaneous combustion. The influences of various pyrite contents on the parameters of spontaneous combustion, such as index gases, temperature and released heat etc., were investigated in this study, Coal samples with different pyrite contents （0 %, 3 %, 5 %, 7 % and 9 %） were made by mixing coal and pyrite. The oxidation experiments under temperature-programmed condition were carried out to test the release rate of gaseous oxidation products at different temperatures. Differential scanning calorimeter （DSC） was employed to measure the intensity of heat release during coal oxidation for various pyrite contents. The results indicate that pyrite can nonlinearly accelerate the process of spontaneous combustion. The coal sample with a pyrite content of 5 % has the largest CO release rate and oxygen adsorption as well. However, the coal sample with a pyrite content of ？ % has the largest rate of heat flow according to the results from the DSC tests. Pyrite contents of 5 %-7 % in coal has the most significant effects on spontaneous combustion within the range of this study. The conclusions are conducive to the evaluation and control for the spontaneous combustion of coal.

Effect of weathering on physico-chemical properties and combustion behavior of an Indian thermal coal

An open air stockpile of conical shape was formed with 3.5 m base diameter and 5 m height using 500 tons of -200 mm size ROM thermal coal on a concrete floor. Coal sample from the stockpile were collected at 30 days interval for analysis of various chemical properties like;proximate analysis, ultimate analysis, gross calorific value (GCV) and various combustion related properties such as ignition temperature, peak temperature, burnout temperature, maximum combustion rate, ignition index, burnout index, combustion performance index and combustion rate intensity index. Experimental results show that, due to weathering of coal fixed carbon decreased from 35.6% to 19.9%, elemental carb on (C) decreased from 46.6% to 28.6%, hydrogen (H) decreased from 3.3% to 2.9% and GCV decreased by up to 55% of original value during 330 days of storage of coal in an open atmosphere. Ash con tent of coal increased form 29.2% to 46.6% due to loss of combustibles. Sulfur (S) of coal was found to get increased from 0.33% to 1.08% during storage. The activation energy of coal combustion increased from 22 kJ/mol to 54 kJ/mol. Variation in combustion parameters signifies that weathering has significant negative impact on coal combustion properties as coal become difficult to ignite.

Combustion characteristics of unburned pulverized coal and its reaction kinetics with CO2

The combustion characteristics of two kinds of unburned pulverized coal (UPC) made from bituminous coal and anthracite were investigated by thermogravimetric analysis under air. The reaction kinetics mechanisms between UPC and CO2 in an isothermal experiment in the temperature range 1000–1100°C were investigated. The combustion performance of unburned pulverized coal made from bituminous coal (BUPC) was better than that of unburned pulverized coal made from anthracite (AUPC). The combustion characteristic indexes (S) of BUPC and AUPC are 0.47 × 10^-6 and 0.34 × 10^-6 %2·min^-2·°C^-3, respectively, and the combustion reaction apparent activation energies are 91.94 and 102.63 kJ·mol^-1, respectively. The reaction mechanism of BUPC with CO2 is random nucleation and growth, and the apparent activation energy is 96.24 kJ·mol^-1. By contrast, the reaction mechanism of AUPC with CO2 follows the shrinkage spherical function model and the apparent activation energy is 133.55 kJ·mol^-1.

Study on the emission characteristics of nitrogen oxides with coal combustion in pressurized fluidized bed

Nitrogen oxides are one of the most significant pollution sources during coal combustion. This experimental study was conducted in a 15 kWth lab-scale pressurized fluidized bed (inner diameter = 81-100 mm, H = 2100 mm) firing with bituminous coals. The effects of operating parameters, including bed temperature (800℃-900℃), operating pressure (0.1-0.4 MPa), excess air level (16%-30%) and flow pattern on NOX and N2O emissions were systematically studied during the tests. During each test the interaction effects of all the operating parameters were properly controlled. The results show that most operating parameters have an opposite effect on NOX and N2O emissions, and the N2O emissions mainly depend on the bed temperature. Increasing the operating pressure can significantly suppress the fuel-N conversion to NOX but enhance its conversion to N2O. With the rise of the excess air level and fluidization number, NOX emissions grow distinctly while N2O emissions remain almost unchanged. Total nitrogen oxide emissions increase with the bed temperature while decrease with the operating pressure.

The Production and Release of CFCs from Coal Combustion

The destruction of the ozone layer in the atmosphere caused by industriallysynthesized CFCs has aroused greatest concerns from the international society, but the CFCs formedfrom burning of coal containing fluorine have not been recognized by the world yet. In the presentstudy, we condensed the gas through cold traps and used the GC-MS to measure the gas composition,and found that the content of CFC-12 in the smog from coal combustion was significantly higher thanthe background value of the local atmosphere. This proves that CFC- 12 is formed in the process ofcoal combustion. This paper discusses a new source of non-synthesized CFCs.

Presenting an engineering classification system for coal spontaneous combustion potential

The phenomenon of coal spontaneous combustion is one of the common hazards in coal mines and also one of the important reasons for the loss of coal in piles and mines. Based on previous researches, different types of coals have different spontaneous combustion characteristics. For coal loss prevention, a measure is necessary for prediction of coal spontaneous combustion. In this study, a new engineering classification system called ＂Coal Spontaneous Combustion Potential Index （CSCPI）＂ is presented based on the Fuzzy Delphi Analytic Hierarchy Process （FDAHP） approach. CSCPI classifies coals based on their spontaneous combustion capability. After recognition of the roles of the effective parameters influencing the initiation of a spontaneous combustion, a series of intrinsic, geological, and mining characteristics of coal seams are investigated. Then, the main stages of the implementation of the FDAHP method are studied and the weight of each parameter involved is calculated. A classification list of each parameter is formed, the CSCPI system is described, and the engineering classifying system is subsequently presented. In the CSCPI system, each coal seam can be rated by a number from 0 to 100; a higher number implies a greater ease for the coal spontaneous combustion capability. Based on the CSCPI system, the propensity of spontaneous combustion of coal can be classified into three potential levels： low, medium, and high. Finally, using the events of coal spontaneous combustion occurring in one of the Iranian coal mines, Eastern Alborz Coal Mines, an initial validation of the mentioned systematic approach is conducted. Comparison of the results obtained in this study illustrate a relatively good agreement.

A new apparatus to establish the spontaneous combustion propensity of coals and coal-shales

Coal and coal-shale both tend to undergo spontaneous combustion under favourable atmospheric conditions. The Wits-Ehac index has been developed in South Africa since the late 1980's to test the spontaneous combustion liability of coal. However, in some cases, the Wits-Ehac index fails to produce tangible results when testing coal-shales. To overcome this problem, a new apparatus has been developed to test carbonaceous materials such as coal and coal-shale under chemical reactions with oxygen and an index has been obtained. This index is called the Wits-CT index. The equipment emulates the influence of oxygen adsorption on carbonaceous material for a period of 24 h without a heating system.The Wits-CT index uses the total carbon content of the sample and the temperature variations obtained from the samples during reaction with oxygen to predict the spontaneous combustion liability. Eighteen samples have been analyzed using both indices and the results are in-line. It was found that coals and coal-shales with higher values of the Wits-CT index are more liable to spontaneous combustion.Further research on different coal-shales is underway in order to establish an extensive database for coal and coal-shales, together with known incidences of self-heating.

Radiant Image Simulation of Pulverized Coal Combustion in Blast Furnace Raceway

The relationship between two-dimensional radiant image and three-dimensional radiant energy in blast furnace raceway was studied by numerical simulation of combustion process. Taking radiant image as radiant boundary for numerical simulation of combustion process, the uneven radiation parameter can be calculated. A method to examine three-dimensional temperature distribution in blast furnace raceway was put forward by radiant image processing. The numeral temperature field matching the real combustion can be obtained by proposed numeric image processing technique.

Emissions of volatile organic compounds and carbonyl compounds from residential coal combustion in China

Emissions of volatile organic compounds （VOCs） and carbonyls from residential coal combustion of five coals with different maturities were studied in a simulated room.The coals were burned in form of honeycomb briquettes in a domestic coal stove,one of the most common fuel/stove combinations in China.Through a dilution system,VOCs and carbonyls samples were collected by canisters and silica-gel cartridges and analyzed by gas chromatography and mass spectrum （GC/MS） and high performance liquid chromatography/ultraviolet （HPLC/UV）,respectively.The results show that the bituminous coals with medium volatile matter content produce the highest emissions while the anthracite yields the lowest.Among the identified carbonyls from the coal smoke,the aromatic compounds （benzaldehyde,2,5-dimethylbenzaldehyde and p-tolualdehyde,m/o-tolualdehyde,benzene,m,p-xylene and trimethyl-benzene） were relatively abundant,which might be due to the molecular structure of the coal.For formaldehyde,aromatic carbonyls and aliphatic alkanes,their concentrations increase up to the maximum values and then decrease with increasing coal maturity.The total carbonyls and VOCs have the same tendency,which was observed for the emission factors of organic carbon （OC）,elemental carbon （EC）,particulate matter （PM） and polycyclic aromatic hydrocarbons （PAHs） detected in the series study.

The catalytic effect of the Na and Ca-rich industrial wastes on the thermal ignition of coal combustion

The catalytic effects of four industrial wastes,namely,the soap residue(SR),brine sludge(BS),calcium carbide residue(CCR),and white lime mud(WLM),on coal thermal ignition were investigated.The acidity of palmitate anion associated with Na+in SR was lower than that of chloride anion combined with Na+in BS,which resulted in an improved the combustion of SR.The acidity of OH-anion combined with Ca2+in CCR was lower than that of CO32-anion combined with Ca2+in WLM,resulting in CCR exhibiting a better catalytic effect on coal ignition.The alkaline metal Na had lower initial ionisation energy than the alkaline earth metal Ca.Therefore,the Na-rich SR exhibited higher catalytic activity on coal ignition than Ca-rich CCR.The ignition temperature of coal with 0.5%SR decreased from 544 to 503°C.

Experiment study of optimization on prediction index gases of coal spontaneous combustion

The coal of Anyuan Mine has the characteristic of easy spontaneous combustion. Conventional method is difficult to predict it. Coal samples from this mine were tested in laboratory. The data obtained from laboratory determination were initialized for the value which was defined as "K". The ratio of each index gas and value of "K", and the ratio of combination index gases and value of "K", were analyzed simultaneously. The research results show that for this coal mine, if there is carbon monoxide in the gas sample, the phenomenon of oxidation and temperature rising for coal exists in this mine; if there is C_2H_4 in the gas sample, the temperature of coal perhaps exceeds 130 °C. If the coal temperature is between 35 °C and 130 °C, prediction and forecast for coal spontaneous combustion depend on the value of Φ(CO)/K mainly; if the temperature of coal is between 130 °C and 300 °C, prediction and forecast for coal spontaneous combustion depend on the value of Φ(C_2H_6)/Φ(C_2H_2) and Φ(C_2H_6)/K. The research results provide experimental basis for the prediction of coal spontaneous combustion in Anyuan coal mine, and have better guidance on safe production of this coal mine.

Spontaneous coal combustion producing carbon dioxide and water

Gas products from the process of coal oxidization and spontaneous combustion have been studied at different temperatures with FTI spectroscopic tests. With temperatures rising to about 30~100 ℃, water and carbon dioxide gas were formed and from about 105~150 ℃, carbon monoxide was produced. Using the DFT B3LYP method with a 6-311G basis set, the reaction system, where spontaneous combustion between coal and oxygen occurs and produces water and monoxide, has been studied, with the geometric configuration for all stagnation points on the potential reaction energy surface optimized. With a frequency analysis and an IRC method, transient formations were tested. Our results indicate that in the reaction of coal oxidization and spontaneous com-bustion producing carbon dioxide and water, oxygen molecules attack carbon atoms of the terminal of the propyl alcohol group on the lateral chain of benzene rings, which causes this propyl alcohol group to produce the acid (-CH2-CH2-COOH) group and water. This acid group continues its break up into carbon dioxide and the (-CH2-CH3) ethyl group. We have come to the conclusion that this water-and-carbon dioxide-production reaction is spontaneous, based on the observation of the energy released by the reaction.

Effects of fractal surface on rheological behavior and combustion kinetics of modified brown coal water slurries

The paper reports the effects of surface fractal structures on the rheological behavior and combustion kinetics of raw brown coal and three modified coal water slurries （CWSs）. The results show that the fractal structures and physicochemical properties of samples are dependent on various modification processes. The apparent viscosities of the coal water slurries increase with increasing surface fractal dimensions （D）, especially with decreasing shear rates. Fur- thermore, it has been proved that the ignition temperatures and apparent activation energies of modified CWSs are lower than that of raw coal water slurry. Compared with the traditional qualitative analysis of the effect of pore structures on CWSs properties, D can more efficiently indicate the quantificational effect of pore structures on the rheological behavior and combustion kinetics of CWSs.

A review on nitrogen transformation and conversion during coal pyrolysis and combustion based on quantum chemical calculation and experimental study

The emission of NOx during coal combustion contributes to the formation of acid rain and photochemical smog,which would seriously affect the quality of atmospheric environment.Therefore,the decrease of NOx is of great importance for improving the efficient utilization of coal.The present review comprehensively summarized the influence factors and mechanisms of migration and transformation of nitrogen during the coal pyrolysis and combustion based on experimental study and quantum chemical calculation.Firstly,in the process of pyrolysis:the occurrence state and transformation of nitrogen were concluded.The influence of temperature,atmosphere,heating rate and catalyst on formation of NOx precursor and nitrogen migration path at the molecular level were summarized;Secondly,during the process of combustion:the influence of temperature,ambient oxygen concentration,physical structure of coal char,catalyst on heterogeneous oxidation of char(N)were summarized;The effects of char surface properties,catalyst and ambient atmosphere on heterogeneous reduction of NOx were also concluded.Based on the quantum chemical calculation,the reaction path of heterogeneous oxidation of char-N and heterogeneous reduction of NOx were described in detail.Current studies focus more on the generation of HCN and NH3,but in order to reduce the pollution of NOx from the source,it is necessary to further improve the process conditions and the optimal formula of producing more N2 during pyrolysis,as well as clarify the path of the generation of N2.Experiments study and quantum chemistry calculation should be combined to complete the research of directional nitrogen reduction during pyrolysis and denitration during combustion.