Spontaneous Combustion Hazard Research Articles

Study on the coupling evolution of air and temperature field in coal mine goafs based on the similarity simulation experiments

In order to prevent the occurrence of the coupling disaster between coal spontaneous combustion and gas explosion, the similarity simulation experiments for coal mine goaf were carried out with or without heat source on the scaled-down experimental platform. Results show that the O2 diffusion speed in strike direction at the air inlet side is significantly greater than that on the air return side, especially under the condition of high ventilation velocity. The higher the ventilation velocity, the greater the risk of coal spontaneous combustion. O2 consumption and thermal convection in the coal spontaneous combustion area make it difficult for O2 to migrate to the deeper goaf. Part of the O2 in air is affected by the chimney effect, rose into the spaces formed by the caving rocks and continuously mixes with the CH4 accumulated in these spaces. Meanwhile, high-temperature area tends to expand in the direction of fresh air leakage in the process of coal spontaneous combustion in the goaf. The formed explosive gases may explode during the migration of high-temperature area, resulting in disaster accidents. Therefore, the temperature change at the location where the air leakage is serious in the goaf should also be monitored in time to prevent the coal spontaneous combustion hazards.

Characterization of Ti powders mixed with TiO2 powders: Thermal and kinetic studies

The fire and explosion risks of metal powders admixed with solid inertants have been extensively investigated for many years. However, it remains unclear why such solid mixtures have high potential fire and explosion risk even when mixed with high percentages of non-combustible solids. This paper investigates how to interpret these risks, from a microscopic perspective, with thermal and kinetic parameters including initial ignition temperature, mass unit exothermic energy, activation energy and risk index of spontaneous combustion. The results show that the initial ignition temperature based on TG (Thermogravimetry) analysis is related to ignition sensitivity, and increased with percentage of admixed solid inertant. The unit mass exothermic energy based on DSC (Differential scanning calorimetry) analysis is related to flame spread velocity. Activation energy and the risk index of spontaneous combustion can be used to explain the reactivity and spontaneous combustion hazard, respectively, of metal powders. We conclude that thermal and kinetic parameters may provide another way to describe the fire and explosion risk of combustible powders, especially for nano metal powders due to the laboratory safety in the normative tests for explosion parameter determination.

Study on the characteristics of coal spontaneous combustion during the development and decaying processes

Large-scale experiments could reflect the real status of coal spontaneous combustion and therefore were conducted to investigate the characteristics of coal oxidation during the development and decaying processes. Three coals with different metamorphic grades were collected and tested to acquire the information including the temperature profiles, movement of the hottest spots, gaseous products, limiting parameters and so forth. The results indicated that the trajectories of hottest spots did not overlap for the development and decaying processes. Meanwhile, compared to the development process, CO productions during the decaying process were higher in the air circumstance and lower in the nitrogen. Coupled with the changes of C2H4 and oxygen consumptions, it could be concluded that both the oxygen concentrations and the stages (development or decaying process) must be taken into account when judging the status of coal spontaneous combustion. Additionally, the discussions about the limiting parameters of the coal spontaneous combustion showed that during the decaying process, the lower limit of coal thickness was smaller, the lower limit of oxygen concentration became less and the upper limit of air leakage turned larger. These results implied that the experience of high temperatures would raise the hazard of coal spontaneous combustion.

Spontaneous Combustion Characteristics of Coal by Using the Simultaneous Thermal analysis–Fourier Transform Infrared Spectroscopy Technique

ABSTRACT Understanding the development process of coal oxidation reactions can aid in preventing coal spontaneous combustion. Oxidation heating experiments on anthracite, bituminous coal, and lignite at different heating rates were carried out using simultaneous thermal analysis–Fourier transform infrared spectroscopy (STA–FTIR). The experimental results show that combining TG, DSC, CO, CO2 and H2O curves in the spontaneous combustion process of coal, the maximum acceleration point Ta and maximum mass loss rate T4 are important for judging the severity of the coal spontaneous combustion hazard. Meanwhile, Ta is more accurate as the ignition temperature point than the ignition temperature T3. As the coal’s rank and heating rate increases, the characteristic temperatures moved to higher temperature. The higher degree of metamorphism has a higher ignition activation energy. Additionally, the reaction models for ignition activation energy between anthracite and bituminous coal changed from three-way transport to random nucleation and nuclei growth with the increase of metamorphic rank, but were not affected by the heating rate.

Coal spontaneous combustion: Examples of the self-heating incubation process

Coal spontaneous combustion continues to pose a significant hazard to mining operations. It is a complex process that ranges from low-temperature oxidation in the normal mine environment to thermal runaway once temperatures exceed 120 °C. At the thermal runaway stage, the coal becomes dry locally after moisture liberation and evaporation and a well-defined hot spot forms. This process takes place over a period of time, which can be referred to as the incubation period. Assessing the spontaneous combustion hazard likelihood has normally relied on the use of laboratory testing to produce index parameters that give a propensity rating. These are often a single value on a relative rating scale, which gives no indication of the nature of the coal self-heating with respect to time as it would occur under various mine site conditions. To overcome this deficiency it is necessary to consider the incubation behaviour of the spontaneous combustion process, particularly as it relates to coal self-heating at initial mine ambient temperatures. This has recently been achieved using adiabatic oven testing, which shows the modifying influences of moisture content, initial start temperature, seam gas content and reactive pyrite content on the low temperature coal self-heating rate. The incubation testing procedure is able to determine whether self-heating can reach thermal runaway and if so in what timeframe this can take place for the environmental conditions present at the mine site.

Modeling and simulation of coal spontaneous combustion in a gob of shallow buried coal seams

The pore and fracture are the path of energy, mass and momentum transfer and conduction in the gob, determine its reasonable distribution is the basis for studying the coal spontaneous combustion hazardous area and migration law of gas products. A discrete fracture-pore model of gob in shallow buried coal seams is being proposed, which is based on the periodic pressure of the overburden, the key strata theory and the compression deformation of the broken rock mass. The model consists of a porous medium media and a discrete fracture network, and calculation equation for the porous medium and the crack width is granted. Taking the Bulianta mine as the background, the model was introduced into FLUENT to simulate the oxygen concentration field, the carbon monoxide concentration field and wind velocity field in the gob. The simulation results agreed well with the measured oxygen concentrations in the field. The spontaneous combustion danger zone and the law of carbon monoxide accumulation in gob are obtained by simulation. In the lower coal seam gob, the maximum width of the spontaneous combustion hazard zone is 108.67m, and the maximum width is 104.94m from the return air side of the gob, and its area is calculated to be 14,645 m2. In the upper coal seam gob, there is basically no spontaneous combustion danger zone that simultaneously meets wind speed and oxygen concentration. However, the oxygen concentration in most areas of the upper gob is between 8% and 18%. In order to prevent the formation of spontaneous combustion danger zone, it is necessary to take corresponding measures for the location with large surface cracks. The concentration of carbon monoxide in the lower gob is greater than the concentration of carbon monoxide in the upper gob. For upper gob, the distribution of carbon monoxide concentration is mainly affected by ground air leakage; for the lower gob, the distribution of carbon monoxide concentration is mainly affected by air leakage of working face.

Experimental Research on the Performance of the Macromolecule Colloid Fire-Extinguishing Material for Coal Seam Spontaneous Combustion

Coal resources are rich in China. They are mainly concentrated in the northwest region. But these resources also make the situation of spontaneous coal combustion hazards most serious in this area. Affected by the arid and semiarid climate, the water resource in this area is relatively scarce. In order to improve the utilization rate of water, a macromolecule colloid for coal mine fire fighting was developed in this research. The main components of the macromolecule colloid and the principle of gel formation were analyzed. The physical and chemical properties of the macromolecule colloid were analyzed by the time of gelation and the water loss. The water content of the macromolecule colloid was as high as 99%. The inhibition performance of the macromolecule colloid on coal fire was studied by the programmed heating device. The experimental results show that the macromolecule colloid has a significant inhibitory effect on the heating rate, oxygen consumption rate, and gas production rate. It inhibits the coal-oxygen composite by increasing the activation energy (Ea) of coal, which also has the effect of wrapping coal to isolate oxygen. The inhibition performance of the macromolecule colloid on coal fire disasters is better than that of water, water glass, loess compound colloids, and other traditional fire-extinguishing materials.

Thermokinetic behavior and microcharacterization of low-rank bitumiteoxidization

As to the continuous consumption of limited high-rank coal resources, low-rank bituminous coal is gradually grabbing more attention to the utilization for power resources and industrial production. However, the high risk of spontaneous combustion of low-rank coal has gradually become one of the most critical hazards worldwide during coal mining, storage, and applications. Thermogravimetry coupled with Fourier transform infrared spectrometry using a nonisothermal program method was employed to investigate the oxidized features of two low-rank coals from Jurassic strata. According to initial, first- and second-derivative thermogravimetric curves, an improved quantitative method for determining the eight characteristic temperatures of the coal oxidation process was proposed. The indexes and implications for the thermal hazard of spontaneous coal combustion during stages I–IV, based on the characteristic temperatures, were analyzed. By examining the functional groups, microcharacterization at each characteristic temperature of the coal was obtained. Three kinetic methods were used to calculate the apparent activation energy ( $$E_{\text{a}}$$ ). The evolution of $$E_{\text{a}}$$ was stage III > stage IV > stage II > stage I. The results demonstrated that the integral mechanism in the four stages described a second-order chemical reaction. Moreover, the pre-exponential factor and $$E_{\text{a}}$$ were positively correlated.

Risk evaluation of coal spontaneous combustion on the basis of auto-ignition temperature

The spontaneous combustion of coal, if not eradicated immediately, may lead to coal ignition and even a full-blown fire. A new method, DSC Inflection Point (DSCIP), was proposed to determine the coal auto-ignition temperature (CAIT). Heat fluxes and kinetic parameters before and after CAIT were comparatively investigated through TG/DSC analysis and mathematical model construction. Meanwhile, the impacts of temperature rise rate and oxygen concentration on CAIT were studied and two indexes representing the hazard and destructiveness of coal spontaneous combustion, respectively, were proposed. The results demonstrated that the heat flux curve of coal spontaneous combustion can be well fitted using Gaussian mixture model. Compared to the oxidation stage, the released heat during the combustion stage was greatly increased. Furthermore, the activation energy became larger and the reaction order decreased to zero when the temperature exceeded CAIT. The study also found that CAIT was enhanced with the increase of temperature rise rates or the decrease of oxygen concentrations. Changes of heat flux, free radicals, and the activation energy proved the rationality and feasibility of the DSCIP method in determining CAIT. Additionally, under the same environmental conditions, lignite had the largest hazard of coal spontaneous combustion and the anthracite had the biggest destructiveness. Both the hazard and the destructiveness of coal spontaneous combustion became stronger as oxygen concentrations increased.

Safety evaluation of combustion-prone longwall mining gobs induced by gas extraction: A simulation study

Abstract Underground methane-rich and combustion-prone longwall mining gobs are usually subjected to the symbiotic threat of spontaneous coal combustion and gas explosion, resulting in significantly economic and environmental impacts. Inappropriate methods or control parameters of gas extraction in combustion-prone longwall mining gobs may trigger a series of secondary accidents, such as the formation of excessive carbon monoxide, and even a gas explosion. Although gob coal–gas interactions induced by gas extraction have been comprehensively investigated previously, few studies are able to quantitatively predict how the symbiotic hazard of gas explosion and spontaneous coal combustion is formed, and guide how to change the design or control parameters for safety and efficiency. In this work, the formation criteria of the coal combustion–gas explosion hazard and a new index of gas extraction safety are proposed to make up for these knowledge gaps. Furthermore, gob gas migration, coal self-heating and their symbiotic risk evolution induced by gas extraction under the different gas control, face mining and ventilation parameters were investigated. The results show that although gas extraction can significantly reduce gob gas emission into the workface, it simultaneously exerts great influence on coal self-ignition and gob safety.

Solid fossil fuels thermal decomposition features in air and argon

Investigation and characterization of coals oxidation is one of the most current problems due to the fact that it may lead to quality loss and spontaneous combustion hazards at mining, storage and utilization. A detailed analysis was performed for thermogravimetric analysis of selection of coals with a wide rank range, different structural characteristics, petrographic and elements composition and origins during experiments in inert and oxidizing environments. It was found that for both the considered environments, the coals thermogravimetric curves retain their space disposition order. Values of the temperatures corresponding to maximal decomposition rates correlate with coal rank as for inert, as for oxidizing environments. For bituminous coals and anthracite, temperatures corresponding to maximal decomposition rate drifted to the zone of higher temperatures in case of experiments in air in comparison with tests in inert. For lignites, they moved to lower temperature intervals. Values of the maximal thermal decomposition rate for bituminous coals well correlate with aromaticity degree. For lignites, the maximal decomposition rates in air and argon have comparable values, whereas for high-rank coals and anthracite they grow in oxidizing environment in comparison with inert one. Kinetic parameters were evaluated for the most common stages of coals mass change during thermogravimetric experiments in air and argon. Kinetic parameters of coals pyrolysis and combustion correlate not only with rank but also with aromaticity degree. For bituminous coals at high temperature intervals at experiments in inert environment, there was found a decrease of activation energy values with rank and aromaticity degree growth. As for corresponding values for bituminous coals tested in oxidizing environment, there was an increase of activation energy with rank and aromaticity degree. Two groups were allocated for coals in context of their interaction with oxygen. The first group included lignites and peat and is characterized by enhanced volatiles release at temperatures over 100°C. The second one contains coals that are prone to oxygen adsorption at low-temperature intervals (up to 300–400°C) (bituminous coals and anthracite). New parameter was proposed for preliminary describing coals group affiliation. This parameter denotes mass gain or decrease for coals tested in oxidizing environment. The complex of kinetic parameters may serve as an additional but informative tool to view at coals oxidation mechanisms. This, presumably, could be performed at simultaneous characterization of thermogravimetric data with heat flow and gas analysis.

Numerical Simulation of Aerogasdynamics Processes in A Longwall Panel for Estimation of Spontaneous Combustion Hazards

The relevance of a solution of the problem of endogenous fire safety in seams liable to self-ignition is shown. The possibilities of numerical methods of researches of gasdynamic processes are considered. The analysis of methodical approaches with the purpose to create models and carry out numerical researches of aerogasdynamic processes in longwall panels of gas mines is made. Parameters of the gob for longwall mining are considered. The significant influence of geological and mining conditions of conducting mining operations on distribution of air streams on longwall panels and effective management of gas emission is shown. The aerogasdynamic model of longwall panels for further research of influence of parameters of ventilation and properties of gob is presented. The results of numerical researches including distribution of air streams, fields of concentration of methane and oxygen at application of various schemes of airing for conditions of perspective mines of the Pechora basin and Kuzbass are given. Recommendations for increase of efficiency of the coal seams mining liable to selfignition are made. The directions of further researches are defined.

Inhibiting effect of [HOEmim][BF4] and [Amim]Cl ionic liquids on the cross-linking reaction of bituminous coal

In order to reduce the hazard of coal spontaneous combustion, the cross-linking reaction between O-containing functional groups of coal should be inhibited. So the inhibitory effect of an ionic liquid (IL) on the cross-linking reaction was studied. The O-containing functional groups change the weight loss and H2O, CO2, CO yields of bituminous coal before and after [HOEmim][BF4] and [Amim]Cl pre-treatment and were detected by Fourier Transform Infrared spectroscopy (FTIR) and Thermo Gravimetric (TG) analysis. The results show that [Amim]Cl has a weaker ability to inhibit the cross-linking reaction of bituminous coal compared to [HOEmim][BF4]. Besides, based on Quantum Chemistry calculation, it was found that the different inhibiting effects of [HOEmim][BF4] and [Amim]Cl are greatly related to their anions and the H linked with C2 atom on the imidazole ring. The H-donor ability of coal will be enhanced by [HOEmim][BF4] leading to a weaker cross-linking reaction of coal.

Development of a spontaneous combustion TARPs system based on BP neural network

Spontaneous combustion of coal is a major cause of coal mine fires. It not only poses a severe hazard to the safe extraction of coal resources, but also jeopardizes the safety of mine workers. The development of a scientific management system of coal spontaneous combustion is of vital importance to the safe production of coal mine. This paper provides a comparative analysis of a range of worldwide prediction techniques and methods for coal spontaneous combustion, and systematically introduces the trigger action response plans (TARPs) system used in Australian coal mines for managing the spontaneous heating of coal. An artificial neural network model has been established on the basis of real coal mine operational conditions. Through studying and training the neural network model, prediction errors can be controlled within the allowable range. The trained model is then applied to the conditions of Nos. 1 and 3 coal seams located in Weijiadi Coal Mine to demonstrate its feasibility for spontaneous combustion assessment. Based upon the TARPs system which is commonly used in Australian longwall mines, a TARPs system has been developed for Weijiadi Coal Mine to assist the management of spontaneous combustion hazard and ensure the safe operation of its mining activities.

Kinetic analysis for spontaneous combustion of sulfurized rust in oil tanks

In order to evaluate the spontaneous combustion hazards of sulfurized rust in oil tanks, one kind of rust was obtained from respiratory valve inner cavity of a crude oil tank in a petrochemical company. The rust was sulfurized in sulfuration experimental apparatus. The production was analyzed by X-ray energy dispersive spectrometry (EDS), scanning electron microscopy (SEM) and then thermo-gravimetric analysis (TGA). The EDS result shows that the main substances are FeS and FeS2 which are liable to spontaneous combustion. The sulfurized rust gives a short length of side and diamond appearance, and a large pore size in structure based on X-ray Diffraction (XRD). The whole oxidation process has three complex stages. The corresponding apparent activation energy values, most probable mechanism functions and pre-exponential factor values were calculated by Madhusudanan–Krishnan–Ninan method and the master plot method. The results indicate that the first and third stages of mass loss are up to the power function mechanism, but the second stage accords with the nucleation and nucleus growth mechanism. The values of apparent activation energy increase successively from the first stage to the third stage. The second stage has the maximum pre-exponential factor value, while the first has the minimum. With the obtained parameters above, the oxidation process of sulfurised rust could be simulated, which would benefit for monitoring and early warning of oil tanks.

Theoretical investigation on the relationship between tail roadway methane drainage and distribution of easily spontaneous combustible region in gob

In order to understand the influence of tail methane drainage pattern on easily spontaneous combustible region in gob, a theoretical qualitative analysis was performed in regards to the spontaneous combustion hazards resulting from drainage position and pressure. Subsequently, the software FLUENT was employed to quantitatively investigate the theoretical solutions by solving the mathematic model established with mass, momentum, component and energy conservation equations. The calculation results demonstrate that methane drainage from alternate exterior roadway to gob can increase the range of easily spontaneous combustible region dramatically; the oxidation zone width increases with increasing drainage pressure, especially near the drainage hole. The rate of temperature rise can accelerate obviously, original high temperature region backs towards gob depth due to enhanced air leakage. The conclusions summarize the negative influence of methane drainage on fire protection in gob and present a method of determining drainage parameters dedicated to prevent spontaneous combustion.

Apparent activation energy for spontaneous combustion of sulfide concentrates in storage yard

In order to evaluate the spontaneous combustion hazard of sulfide concentrates in storage, three different kinds of sulfide concentrates (sulfur-rich sulfide concentrate, iron sulfide concentrate and copper sulfide concentrate) were obtained from a storage yard in Dongguashan Copper Mine, China. The reaction processes at different heating rates of 5, 10, 15, 20, and 25 °C/min in air flow from ambient temperature to 1 000 °C were studied by TG-DTG-DSC analysis. By the peak temperatures of DTG curves, the whole reaction process for each sample was divided into different stages, and the corresponding apparent activation energies were calculated by the Ozawa-Flynn-Wall method. It is found that the reaction process of each sample is considerably complex; the apparent activation energy values change from 36 to 160 kJ/mol in different temperature ranges; sulfur-rich sulfide and iron sulfide concentrates have lower apparent activation energy than copper sulfide concentrate below 150 °C; so they are more inclined to cause spontaneous combustion at ambient temperature.

A comprehensive hazard evaluation system for spontaneous combustion of coal in underground mining

The traditional identification of spontaneous combustion hazard of coal in China is unilateral and inaccurate, which is singly attributed to the properties of coal. Based on current research findings on coal fires, we've reconsidered the influential factors that lead to the spontaneous combustion of coal seams, and have ultimately built a comprehensive system for evaluating the hazard of spontaneous combustion. Applying holistic scoring method and Analytic Hierarchy Process, we integrated all of the factors into a quantitative model, and classified the ratings into five levels. Additionally, a Visual Basic 6.0 based application program has been developed to assist the evaluation, as well as three domestic colliery cases with varying fire hazards have been tested. Through practical application, the hazard evaluations have proved to be objective and valid, so the system is applicable to Chinese coal mines.

A NEW DIMENSION TO STUDIES OF SPONTANEOUS COMBUSTION OF COAL

Spontaneous combustion of coal is a hazard frequently encountered in coal mining. It has been researched extensively but the actual mechanism by which it occurs is not completely understood. Under suitable conditions, the spontaneous combustion hazard manifests itself in all major aspects of coal mining, namely, underground coal mining, surface mining, stockpiling of coal at the pithead, power stations and ports, during sea-borne transport and stacking of reject material. In underground coal mining, the most difficult situation occurs when spontaneous combustion takes place in the presence of an inherently explosive atmosphere which may promote a local spontaneous combustion incident into an explosion hazard involving the entire mine. Such an incident at the Moura mine in Central Queensland resulted in greater pressure on mine operators for the safe management of spontaneous combustion in their underground coal mines than ever before. As not all coal mines are equally liable to spontaneous combustion due to the different propensities of different coals to self-oxidation, the risk assessment for spontaneous combustion is a primary requirement for the development of a spontaneous combustion management plan for a mine. Once the coal seam has been classified into various categories of spontaneous combustion risk, it is necessary to determine a fire ladder for the coal seam. This should be followed by detailed planning of the mine layout including the mine ventilation system and a spontaneous combustion monitoring program before devising a spontaneous combustion management plan as required by the Coal Mines Underground Regulations 1999. In this paper, a case history is presented which includes the development of a spontaneous combustion management plan for a high-risk coal seam in a greenfield site, from the risk assessment stage to the outbreak stage of a spontaneous combustion incident. It is concluded that the risk assessment, evaluation of the fire ladder, comprehensive monitoring by a tube bundle system and trending of carbon monoxide/oxygen deficiency ratio (Graham's ratio) and CO levels were the main elements in an effective spontaneous combustion control system for the mine.

堆積微粉炭の自然発火と炭塵雲の爆発

Danger of spontaneous combustion and dust explosion at coal handling and utilizing facilities is outlined.Spontaneous combustion is a resultant phenomenon of competition of the rates of heat generation and heat loss. Influence of factors on the initiation is described on the basis of the thermal explosion theory.Explosion characteristics of dust clouds are described in relation to the assessment and protection-suppression measures.It is emphasized that decreasing particle size increases the hazard of spontaneous combustion and dust explosion but there are little data for the assessment for finely pulverized coals.