Spontaneous Combustion Tendency Research Articles

Heat Effect of Oxidation of Aliphatic Hydrocarbon Groups on the Piecewise Characteristics and Spontaneous Combustion Tendency of Coal

Heat Effect of Oxidation of Aliphatic Hydrocarbon Groups on the Piecewise Characteristics and Spontaneous Combustion Tendency of Coal

Study on Pore and Fissure Structure Characteristics of Deep Soft Coal Rock

The microscopic pore and fissure structure is the key factor affecting the exploitation, storage, and migration of coalbed methane and coal spontaneous combustion tendency. For further research of the microstructure of deep soft coal rock, such as pores and fissures, the coal samples from the Yangdong mining area were qualitatively and quantitatively analyzed in terms of morphological characteristics, pore shape, pore specific surface area, pore volume, and pore diameter by a scanning electron microscope (SEM) and a low-temperature liquid nitrogen adsorption experiment. The results show that there are three major categories and five minor categories of pores with different genetic types, including metamorphic pore, exogenous pore, and mineral pore, and there are endogenous fissures, exogenous tensile fissures, and exogenous shear fissures developed in the coal body. According to the results of the low-temperature liquid nitrogen adsorption experiment, the hysteresis curves of coal samples can be divided into two types. The I type curve produces a loop. There is a “hysteresis loop” which is obvious, and there is an inflection point that is not obvious. The pore system is mainly composed of open pores. The II type curve has no adsorption back line and no obvious inflection point. The pore structure is mainly composed of an impermeable hole closed at one end. The BET specific surface area of coal samples ranges from 0.2810 to 4.7569 m2/g, with an average of 1.27984 m2/g. The BJH pore volume ranges from 0.002864 to 0.007377 cm3/g, with an average of 0.0041246 cm3/g. The average BJH pore diameter of coal samples ranges from 4.3935 to 20.1501 nm, with an average of 16.0313 nm. The pore specific surface area of coal is mainly contributed by micropores, and the transition pores contribute the most to pore volume. The distribution of pore volume in each pore section of a coal sample has the rule that the transition pore is larger than the micropore, and the micropore is larger than the mesopore, and the maximum ratio is 66.2%. The distribution of pore specific surface area has the rule that the micropore is larger than the transition pore, and the transition pore is larger than the mesopore. The maximum ratio is 91.2%.

Experimental investigation on microstructure evolution and spontaneous combustion properties of aerobic heated coal

Since coal oxidation and heat accumulation are prerequisites for coal spontaneous combustion, it is of great significance to study the characteristics of coal oxidation and heat accumulation to grasp characteristics of spontaneous combustion of residual coal and to locate the area with coal spontaneous combustion in goaf. In this study, three types of lignite and bituminous coal with a strong spontaneous combustion tendency were selected to prepare raw coal samples and aerobic heated coal samples. Then ultimate and proximate test, core pore and surface area test, SEM test, TG test, FTIR test and temperature-programmed test were undertaken, respectively. After coal undergoes aerobic heated progress, its surface micro-cracks, internal pore volume and specific surface area surge, and the active functional groups vary significantly; the contents of C and H fall, and that of O rises. These changes are conducive to coal oxidation. The temperatures corresponding to the first peak points in the DTG curves for the aerobic heated coal samples are lower than those for the raw coal samples, which indicates that the aerobic heated coal samples enter the self-oxidation acceleration stage earlier. Besides, according to the results of the temperature-programmed experiment, the cross-point temperatures of CO generation for the aerobic heated coal samples and the raw coal samples are 91 °C, 82 °C and 76 °C, respectively. A comprehensive analysis discloses that the aerobic heated coal samples are more likely to combust spontaneously in the initial stage of oxidation and to generate more CO. The heat dissipation zone, oxidation zone and suffocation zone were divided by monitoring the oxygen concentration changes in goaf at different stages on 5–20101 working face. It is determined that the oxidation zone of the goaf is 71–152 m on the inlet side and 59–128 m on the return side.

Prediction of spontaneous coal combustion tendency using multinomial logistic regression

Spontaneous combustion of coal is a complex underground mining disaster, which mainly threats mine safety and efficiency. Several factors usually cause spontaneous combustion of coal, such as gas concentration, ventilation and coal properties. In this study, spontaneous combustion tendencies of coal mines were predicted considering the effective parameters for an underground coal mine in Turkey. Multinomial logistic regression, a multivariate statistical technique, was applied. Gas concentrations (CH4, CO, O2) and air velocity were defined as factors affecting spontaneous coal combustion. Fire hazard levels of the coal mines were determined as ‘normal situation’ and ‘potential combustion’. It was observed that CH4 and CO variables and CH4 × CO interaction were effective in the formation of clusters. The results indicate that Mine I is more liable to spontaneous combustion than Mine II and Mine III. At the same time, the effects of variations in factors are examined in the study.

Optimization Studies on Biological Desulfurization of Sulfide Ore Using Response Surface Methodology

This research aimed to optimize the experimental conditions of biodesulfurization of sulfide ore and to evaluate the flame-retardant effect after desulfurization under optimal conditions. Six experimental factors were determined: particle size of ore sample, ambient temperature, rotary speed of the shaking table, volume of bacteria liquid, concentration of leaching aid (Tween80), and pH value of acidizing ore sample. Desulfurization efficiency was used as the main characterization index of the desulfurization effect in optimization studies. Particle size of ore sample, rotary speed of the shaking table, and volume of bacteria liquid inoculated were selected as significant factors by a Plackett–Burman experiment. Modeling, optimization, and analysis of the interactive effects of these factors, notably between particle size and bacteria liquid, were performed using a Box–Behnken design with response surface methodology. The optimum operating conditions were found to be: particle size of 120 to 140 mesh, rotary speed of 175 rpm, and bacteria liquid of 111 mL. Under these conditions, a significant rise of 8.1% was seen in 5-day average desulfurization efficiency. The 5-day oxidation weight gain rate of desulfurized ore was 2.73%, while that of the control group was 4.78%. The results show that, after optimized desulfurization, the surface oxidizability and spontaneous combustion tendency of the ore are reduced.

Evaluation of the spontaneous combustion tendency of corrosion products in oil tanks based on TOPSIS methodologies

Spontaneous ignition of the corrosion products of an oil tank may lead to an explosion. To evaluate the spontaneous combustion tendency of corrosion products in oil tanks, the thermodynamic characteristic parameters of the corrosion products of oil tanks were obtained by thermogravimetry as evaluation indexes. Then, the objective weights of these indexes were calculated via the CRITIC method, and TOPSIS theory was used to comprehensively evaluate the spontaneous combustion tendency of corrosion product samples from oil tanks. The results show that the weight of the mass loss rate at the ignition point is the highest, and the apparent activation energy of the corrosion products is not the decisive factor affecting their spontaneous combustion tendency. As such, the spontaneous combustion tendency of the corrosion products of oil tanks cannot be based on only the apparent activation energy. The spontaneous combustion tendency of the corrosion products of a gasoline tank is lower than that of a diesel tank. The higher the heating rate is, the stronger the tendency of spontaneous combustion will be.

Oxidation Characteristics of Functional Groups in Relation to Coal Spontaneous Combustion.

To investigate and better understand the mechanism of coal spontaneous combustion, the distributions, evolution, and oxidation characteristics of functional groups in different coal samples were characterized using in situ Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) experiments. The macroscopic characteristics of coal spontaneous combustion in relation to functional groups were also analyzed using the thermogravimetric/differential scanning calorimetry–FTIR coupling technique. The experimental results indicated that −OH was the most active groups of coal spontaneous combustion. It not only could react with the absorbed oxygen spontaneously but also found to be the main product of the chemisorption. Consequently, −OH was believed to contribute most both for the loss and increase of coal mass during the process of spontaneous combustion. Aliphatic hydrocarbons were the main components to form −C–O–O• and could be further oxidized into C=O. However, reactions between aliphatic hydrocarbons and oxygen were nonspontaneous. EPR experiments suggested that the tendency of coal spontaneous combustion acutely depended on the stability and survival time of free radicals. The more the stable and longer survival time of free radicals are, the lower the tendency of coal spontaneous combustion is.

Study on the Spontaneous Combustion Tendency of Coal Based on Grey Relational and Multiple Regression Analysis.

The correlation between the spontaneous combustion tendency of coal and its properties are of great importance for safety issues, environmental concerns, and economic problems. In this study, the relationship between multiple parameters, different from the previous single parameter, and the spontaneous combustion tendency was analyzed. The comprehensive judgment index (CJI), which indicates the tendency of coal spontaneous combustion, was obtained for samples collected from different mines. The CJI was measured by the cross-point temperature and had a negative correlation with the spontaneous combustion tendency. Physical pore structures and chemical functional groups were characterized based on cryogenic nitrogen adsorption and Fourier transform infrared spectroscopy measurements, respectively. For analyzing the effect of coal properties on the spontaneous combustion tendency, the grey relational grade was determined by the grey relational analysis between the CJI and the pore structures and functional groups of coal. The grey relational grade of the benzene substituent with CJI had a maximum of 0.8642, and the macropores had the minimum, 0.4169. The higher the gray relational grade was, the more relevant the spontaneous combustion tendency was, indicating that the benzene substituent was the most relevant. To better predict the spontaneous combustion tendency, the average pore diameter, hydroxyl, methyl, methylene, and benzene substituent with a high grey relational grade were selected. Finally, the multiple regression prediction model of CJI was established. The R squared coefficient, significance level, F-distribution, t-distribution, collinearity diagnosis, and residual distribution of the model met the requirements. In addition, two coal samples were selected to verify the spontaneous combustion tendency model. The relative errors between the predicted CJI value and the experimental CJI value were 1.42 and 4.25%, respectively. These small relative errors verified the reasonableness and validity of the prediction model.

Experimental investigation of temperature distribution and spontaneous combustion tendency of coal gangue stockpiles in storage.

In this study, an experimental model about the coal gangue stockpiles in semi-open storage was developed. According to the model's requirements, the corresponding coal gangues were piled up in the factory building, the heat source and collection points were arranged, and the four operating temperatures were selected from 70 to 350 °C for heating. A series of fire experiments concerning the temperature distributions of the coal gangue piles were conducted systematically. The spontaneous combustion tendency of coal gangue samples under kinds of the four heat sources was analyzed using scanning electron microscope (SEM), thermal gravity analysis, and differential thermal gravity (TG-DTG). Under the action of thermal damage, the surface of micropores in coal gangue becomes rough. Heat accumulation is, in nature, most likely to occur near 0.1~0.4 m away from the heat source of coal pile. Simultaneously, on each of the measured flat layers, the greater the horizontal distance from the heat source is, the lower the heated temperature of gangues is, and the lower the temperature change rate is, indicating that the horizontal heat conduction is also gradually weakened. The experimental model provides an empirical basis for studying the distribution of temperature field in the depth of gangue pile and kinetics reaction mechanism of spontaneous combustion.

Dissect the capacity of low-temperature oxidation of coal with different metamorphic degrees

Radical chemistry is a major factor affecting the low-temperature oxidation of coal, which plays a critical role in the coal oxygen complex reaction. However, the characteristics of radical for judging its ability to oxidize are still poorly understood. The electron paramagnetic resonance (EPR) spectroscopy has emerged as a powerful approach in studying the behavior of radicals. So the EPR spectra of coal low-temperature oxidation were investigated by in-situ EPR spectroscopy. Analysis of the initial temperature of CO evolution and its increment and crossing-point temperatures (CPTs) of coal reveal that the oxidizability of low-rank coal is more prominent than others. The EPR linear profile of low-rank coal has a well temperature dependence and a large g factor, an indicator of oxidizability. However, it is not consistent with the increasing trend of the absolute content of radical concentration. This study provides a novel approach to quantify the oxidizability of coal using the relative growth rate R of radials, and the results are consistent with the coal spontaneous combustion tendency. The main objective of the current study is to obtain a comprehensive understanding of low-temperature oxidation of coal with different ranks.

Exploring thermokinetic behaviour of Jurassic coal during pyrolysis and oxidation

The thermokinetics and gaseous products of Jurassic coals collected from northern Shaanxi, China at four different heating rates were determined via thermogravimetry-Fourier transform infrared spectroscopy experiments. The results showed that the trends of thermogravimetric pyrolysis and oxidation curves were divided into three stages and five stages, respectively, and presented the same variations. As the heating rate increased, their curves moved the high temperature. In addition, the characteristic temperatures indicated that the coal samples had a relatively significant spontaneous combustion tendency. In the stage of water evaporation and gas desorption, the rates of formation of CO and CO2 went up promptly, and gaseous water produced during the oxidation reaction began to increase, but its generation rate was lower than that in the stage of oxygen absorption and mass increase. The apparent activation energy (Ea) showed a characteristic jump with increasing temperature, and the optimal mechanism function was chosen by integration and differentiation. Moreover, the relationship between Ea and lnA was linear, which indicated that there was a kinetic compensation effect.

Dual-hazard control mechanism of burst-prone and spontaneous combustion coalface considering effect of retreat speed

For longwall top coal caving coalface characterized by strong rock burst tendency and high risk of spontaneous combustion, retreat speed is one of the key factors to induce the rock burst and spontaneous combustion of residual coal in goaf. A high retreat speed may lead to violent roof activity and impact dynamic load, which aggravates the tendency of rock burst. A low retreat speed may lead to spontaneous combustion of residual coal in goaf due to long time exposure to aerobic environment. In this study, the characteristics of overburden stress distributions, energy release laws, and the evolving laws of the oxidation and temperature rising zones in the goaf under different retreat speeds are analyzed. The spontaneous combustion coefficient of residual coal Qs is defined. What is more, the dual-hazard control mechanism of rock burst and spontaneous combustion of residual coal considering effect of retreat speed is put forward. The results show that the energy release of the stope with low retreat speed is stable, while that with high retreat speed appears obvious fluctuation phenomenon. With the increase of retreat speed, the relative stress concentration coefficient increases exponentially, while the spontaneous combustion coefficient of residual coal in goaf decreases exponentially. According to the curve of retreat speed and relative stress concentration coefficient and curve of retreat speed and spontaneous combustion coefficient, the reasonable operating interval and optimal operating point of retreat speed in strong rock burst tendency and easy spontaneous combustion coal seam can be determined.

Reaction characteristics and kinetics of the low-temperature oxidation and weight gain of coal

ABSTRACT The study of coal oxidation and weight gain at low temperatures is helpful in understanding the spontaneous combustion of coal and its mechanisms. In this study, we comprehensively investigated the low-temperature oxidation properties and the oxygen adsorption weight gain kinetics of Chinese Shenhua subbituminous coal (SH). Using thermogravimetric experiments with various particle sizes, heating rates, and coal ranks, the oxygen adsorption mass gain stages of coal oxidation at low temperatures were investigated. According to the results of these experiments, as the particle size of SH coal decreased, the dry cracking temperature (T1) decreased, the ignition temperature (T2) changed slightly, the weight gain increased, and the spontaneous combustion tendency increased. The three-dimensional diffusion model (Zhuralev-Lesokin-Tempelman) (D5) is suitable for SH coal samples with different particle sizes. A higher heating rate is not conducive to the formation of carbon-oxygen complexes. The D5 model can also be used to fit the kinetics of the SH coal samples at 5 K/min, 10 K/min, and 30 K/min. Unlike the SH coal samples, the Shengli lignite (SL) and Tavan Tolgoi anthracite (TT) samples fit the three-dimensional diffusion model (Jander) (D3). Based on the fitting results of the Achar differential method and the Coats-Redfern integral method, the activation energy E of the low-temperature oxidation weight gain of the coal samples decreases with decreasing coal particle size and with increasing oxidation rate and coal rank.

Study on Preparation and Performance of Calcium Carbide Slag Foam for Coal Mine Disaster Reduction and CO2 Storage

In the process of coal mining, coal spontaneous combustion in the gob area poses serious threats to the safety of mine production. Besides, coal combustion also produces a large amount of greenhouse gas CO2, which triggers a series of ecological environment problems. In order to solve these two problems, the idea of mineralizing CO2 with the alkaline industrial waste calcium carbide slag and then sending the foams to the gob area for coal spontaneous combustion prevention was proposed in this study. First, the effects of calcium carbide slag foaming under the action of different types of foaming agents, including sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), polyoxyethylene fatty alcohol sodium sulfate (AES), and octyl decyl glucoside (APG0810), were tested and analyzed. Furthermore, the effects of water glass (WG), sodium alginate (SA), polyvinyl alcohol 1799 (PVA), cement, and silica micro-powder on the foam stability were analyzed by investigating the stability coefficient and drainage volume of calcium carbide slag foam. On this basis, the configuration scheme of calcium carbide slag foam with high efficiency and good stability was determined. The experimental results on CO2 mineralization and coal spontaneous combustion inhibition reveal that the prepared calcium carbide slag foam can rapidly absorb CO2. Moreover, after calcium carbide slag foam treatment, the spontaneous combustion tendency of the coal sample is significantly reduced.

Thermodynamic Model and Kinetic Compensation Effect of Spontaneous Combustion of Sulfur Concentrates

The spontaneous combustion of the sulfur concentrate is the main hazard faced in ore storage bins. To understand the thermodynamic characteristics of spontaneous combustion of the sulfur concentrate and test whether the kinetic compensation effects are present in the spontaneous combustion process of the sulfur concentrate, typical sulfur concentrate samples were selected as the research object, and thermogravimetric experiments were carried out under an air atmosphere at heating rates of 5, 10, and 15 K/min. On this basis, the contributions of different reaction models to the mass change during the spontaneous combustion of the sulfur concentrate, as well as the thermodynamic model and kinetic compensation effect, are analyzed. The results show that solid-phase combustion contributes the most to mass loss among different mechanisms of the reaction between the sulfur concentrate and oxygen. The contributions of reaction models to mass loss are affected by the different heating rates, and the contribution of solid-phase combustion to mass loss increases with increasing heating rates. The Malek method is used to obtain the kinetic model of the spontaneous combustion of the sulfur concentrate, and its mechanism function changes from a chemical reaction model to a three-dimensional diffusion model. There is a kinetic compensation effect in the spontaneous combustion process of the sulfur concentrate, and the level of the kinetic compensation line may be one of the bases for distinguishing the spontaneous combustion tendency of the sulfur concentrate.

Investigation of the effect of reactor size on spontaneous combustion properties of coals with different coalification degrees

The spontaneous combustion of coals is a natural phenomenon that causes serious losses in the mining sector as well as environmental problems as a result of greenhouse gases released during spontaneous combustion of coal. In this study, for the first time, the effect of coal reactor size on the spontaneous combustion tendency parameters (crossing point temperature (CPT), average heating rate (AHR), and liability index (IFCC)) of the coal samples having two different ranks (bituminous coal and lignite) have been searched by the use of crossing point method. According to the results obtained from the reactors manufactured in three different sizes (R-1, R-2, and R-3), ignition temperatures of coal samples were 161–172 °C for bituminous coal and 143–175 °C for lignite; average temperature rise was 0.92–1.45 °C min−1 for bituminous coal and 1.47–2.98 °C min−1 for lignite; spontaneous combustion risk index was determined in the range of 5.61–8.88 min−1 for bituminous coal and 8.39–20.79 min−1 for lignite. As the reactor size decreased, spontaneous combustion tendencies of lignite samples increased, and no significant change was observed in bituminous coal samples. It has been found that the effect of reactor size on the spontaneous combustion of coal gives significant results in low-rank coals but has no effect on high-rank coals.

A comprehensive study on the effect of moisture content on coal spontaneous combustion tendency

There are several phenomenons for polluting the environment, especially in coalfields; which coal spontaneous combustion is one of them. The moisture content is one of the intrinsic characteristics of coal, which has an important role in the occurrence of this phenomenon. Therefore, this research predicts the coal spontaneous combustion tendency based on moisture content. The percentage of moisture content is a very important parameter on the occurrence of this process; so far a conclusion about the effect of moisture content on coal spontaneous combustion, a comprehensive study was done. 55 coal samples with different percentage of moisture content for the training of overall underground coalfields in Iran were collected and the CPT test method for each coal sample was carried out. Then, the method of regression analysis was used for modeling and predicting the coal spontaneous combustion tendency. The results show, the coal sample undergoes oxidation most rapidly when the moisture content supply is about under 20%, and it can reduce coal spontaneous combustion in excessed of 20%, because when moisture is present in excessed of 20%, the heat released by oxidation is used to evaporate the moisture. For validation and testing, 15 coal samples of another coalfield were collected and the CPT test method for each coal sample was carried out, and the results of the test method were compared by the regression equation. The results obtained from the models show that a good appropriate prediction has been done for determining the coal spontaneous combustion tendency by regression analysis.

Prediction and prevention of spontaneous combustion of coal from goafs in workface: A case study

Aiming at forecasting and preventing remnant coal spontaneous combustion in goaf, this thesis conducts determination on the spontaneous combustion tendency of coal seam, simultaneous thermogravimetry and differential scanning calorimetry(TG-DSC) analysis on coal samples, optimization on index gas of spontaneous combustion, and division of spontaneous combustion “three zones” with O2 concentration as the indicator. The conclusions are as follows: the spontaneous combustion period of remnant coal is 1–3 months, so the coal seam is easy to spontaneous combustion. CO and C2H4 are selected as the index gas for forecasting spontaneous combustion in No.12 coal seam and No.12-up coal seam because of their regular evolution tendency in concentration changing with coal temperature and easiness in detecting. As to the spontaneous combustion “three zones” divided by O2 concentration, the depth and width of both heat dissipation zone and left side oxidation zone in the goaf is slightly increased when the “Y” type ventilation is adopted, comparing with the “U” type ventilation, and the asphyxiation zone is extended to about 200 m behind the workface. Pressure balance method and ground fissure sealing are proposed for spontaneous combustion prevention and extinction, which offers some practical experiences for adjacent mine or similar coal seam.

Analysis of Coal Spontaneous Combustion by Thermodynamic Methods

ABSTRACT In order to better determine the spontaneous combustion tendency of coal, two types of heating rate methods, namely, single and multi-scan methods, commonly used in the field of spontaneous coal combustion are summarized based on a thermal analysis kinetics method. The single scanning rate method mainly included the Coats–Redfern, Achar, universal equation, MacCallum–Tanner, Madhusudanan–Krishnan–Ninan, and non-isothermal thermogravimetric and differential scanning calorimetric methods. The multi-scan rate method mainly included the Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, Starink, and Friedman methods. For each of these methods, their respective theoretical basis, advantages, and disadvantages were described. In addition, the Popesc and Malek methods were used to determine the optimal reaction kinetic model. In view of the compensation effect, the invariable activation energy E inv and the pre-exponential factor lnAinv at different heating rates were determined. For each method, the oxygen weight gain stage of Shaanxi bituminous coal was selected as an example interpretation.

Spontaneous Combustion Tendency in Bagasse Stockpiles: Biotic and Abiotic Responses Relation

Microbial activity in stockpiled bagasse was investigated using: (1) acidity levels within the stockpile and (2) detected amounts of flammable volatile organic compounds (VOCs) above the stockpile, as potential factors to identify their impacts on spontaneous combustion. Temperature, moisture content, microbial activity (estimated by the number of spores obtained from plate counting), pH value, amount of acetic acid at different depths (0.5, 1.0, 1.5, 2.0 and 2.5 m from the stockpile surface) and the flammable VOCs above the stockpile surface after different storage periods (100, 150, 200 and 250 days from the first stockpiled day) were investigated. The temperature and moisture content of the stockpile were in the ranges 50–65 °C and 50–65%, respectively. A decrease in the pH value (pH 4 to 3) after longer storage periods was observed, which was related to the decreases in mesophile spores (1 order of magnitude) and thermophiles spores (1 order of magnitude) within the stockpile. Increases were observed in the temperature (5–10 °C), in the production of acid (3.6% m/m on dried bagasse) and of flammable VOCs, such as ethanol in the range 118–130 µg per cubic meter of air and others above the stockpile. Acid production increased the flammable VOCs, the temperature and the moisture content, suggesting that these parameters have the potential to be used for monitoring the stockpile characteristics. The acidity level is particularly useful since it is easy to measure.