Spontaneous Combustion Propensity Research Articles

Data-driven revision of coal spontaneous combustion risk classification for prevailing methods used in Australian mining industry

Spontaneous combustion poses persistent hazards in underground coal mining operations, threatening personnel and operations. To assess this risk, in Australia small-scale laboratory tests are being conducted using Adiabatic Oxidation (R70), Crossing Point (CPT) and Minimum Self-Heating Temperature (SHTmin) methods. The intrinsic spontaneous combustion propensity classification (ISCP) utilizes R70 results to establish a risk rating. This risk ranking provides guidance on the severity of spontaneous combustion ranging from low to extremely high, aiding in the implementation of preventive measures. However, The ISCP lacks supporting literature, and there is no existing literature on correlation between laboratory tests or risk matrix for CPT. This paper presents the results of a Spearman correlation study among R70, CPT and SHTmin based on a large historical database (n = 318) showing that only R70 and CPT can be used to reliably rank spontaneous combustion risk. It was found that CPT is strongly correlated with R70 (ρ = -0.8875), but SHTmin shows a weaker correlation with R70 (ρ = -0.7265). The hierarchical clustering analysis resulted in a revised risk ranking: Low (R70 < 0.4 °C/h, CPT > 156 °C), Medium (0.4 °C/h < R70 < 3 °C/h, 132 °C < CPT < 156 °C), High (3 °C/h < R70 < 11 °C/h, 102 °C < CPT < 132 °C), and Very High (R70 > 11 °C/h, CPT < 102 °C).

Coal Secondary Oxidation Thermodynamic Behaviors Affected by Pre-Heating at Different Oxygen Concentrations

ABSTRACT To investigate the impact of pre-oxidation oxygen concentration on the propensity of coal spontaneous combustion, thermogravimetric analysis was used to obtain thermal characteristic curves for raw coal and pre-oxidized coal with five different oxygen concentrations. The coal spontaneous combustion process was categorized into three phases: water-loss and weightlessness, oxygenation and weight gain, and combustion phase. It reveals that characteristic temperature points of oxidized coal were lower than that of raw coal, and pre-oxidation plays a role in enhancing the coal oxidation to varying degrees. During the phase of oxygenation and weight gain, the activation energy of oxidized coal decreases from 44% to 84% of that of raw coal. The positive effect on coal secondary oxidation is initially reduced, then increased with the increasing of pre-oxidation oxygen concentration. The sample pre-oxidized with 15% oxygen concentration is the most easily spontaneous combustion, and the total activation energy of its oxidation reaction is 57% of that of the raw coal, the characteristic temperature of which lowered by an average of 7.4°C. It will be conducive to revealing reignition mechanism of coal and preventing coal secondary spontaneous combustion.

Determinants of prioritised influencing factors on coal spontaneous combustion propensity – A Fuzzy-Delphi-geometric mean analytic hierarchy process

One of the most common hazards in coal mines is spontaneous combustion, a significant cause of coal loss in underground mines. Specifying a suitable approach for the determinants of prioritized influencing factors on coal spontaneous combustion propensity is challenging. There needs to be more research that directly addresses the uncertainty of human judgement in assessing the factors influencing coal propensity to self-combustion. This is the first attempt to address the aforesaid problem by ranking the factors influencing spontaneous coal combustion. The ranking was performed using the Fuzzy Delphi Analytic Hierarchy Process (FDAHP) with two variants of the Geometric Mean Method (GMM), which enabled the analysis of both quantitative and qualitative data related to the rating of each factor influencing spontaneous combustion risk and the relative importance of each factor related to coal seam characteristics, geological aspects, mining practices, and environmental aspects. The results show that ‘GMM as a whole’ is a comparatively best-aggregating approach to the AHP process. The presented case study of using this methodology in underground mine conditions confirms its efficacy in assessing susceptibility to coal spontaneous combustion. This mainly concerns the pace and reliability of the obtained results and the determination of risk evaluation system based on intrinsic and extrinsic factors influencing susceptibility to spontaneous combustion of coal. Finally, this systematic technique was validated in the research area by utilizing spontaneous combustion incident that occurred in Jharia and Raniganj coalfields, which revealed rather excellent concordance in Jharia and Raniganj coalfields. The study can be used in mine planning to develop engineering data to select the operational parameter and prevent the initiation of spontaneous heating by identifying intrinsic factors and optimizing extrinsic factors.

Experimental study of microbial desulphurisation of sulphide ores combined with gel foam retardants to prevent spontaneous combustion

ABSTRACT The purpose of this study was to reduce the propensity for spontaneous combustion of sulfide ores through the combined action of microorganisms and inhibitors and to assess the flame retardant effect. The sulfide ore samples were first pretreated with biodesulphurisation. The samples were then sprayed with a foam blocker to prevent re-oxidation of the ore and further reduce the spontaneous combustion tendency of the sulfide ore. The experimental results showed that the highest desulfurization rate of 24.26% was achieved for ore samples with a particle size between 140–160 mesh. A combination of 0.05% hexadecyltrimethylammonium bromide (CTAB), 0.11% lauramidopropyl betaine (LAB-35), 0.13% sodium alcohol ether sulfate (AES), 0.11% propanetriol foam stabilizer, 7% water glass and 3% sodium bicarbonate arresting agent was the most effective. The oxidation weight gain of the desulphurised ore sprayed with the resist was only 3.16%. The oxidation weight gain of the desulphurised ore without the addition of the resist was 3.98% and the oxidation weight gain of the undesulphurised ore was 6.01%. This method is effective in reducing the tendency of spontaneous combustion of sulfide ores.

Spontaneous combustion of carbonaceous shale at an iron ore mine, South Africa

Spontaneous combustion during coal mining operations is a major problem that affects the health and safety of workers and causes environmental problems. The phenomenon is associated with the presence of coal, coal shale, and pyrite. In 2020, a premature detonation incident occurred at an iron ore mine where the waste material contains black carbonaceous shale units known to be associated with pyrite. The spontaneous combustion propensity and properties of samples of the black carbonaceous shales from the mine were examined and compared with samples from the Witbank Coalfield. The spontaneous combustion liability indexes of these samples were correlated with X-ray fluorescence (XRF) and proximate and ultimate analyses using linear regression. The Wits-Ehac Index classification results show that the samples were between medium and high risk. The linear regression analysis showed very poor correlations between the Wits-Ehac Index results and the XRF and proximate and ultimate results. The most valuable relationship found is between the presence of relatively high sulphur (greater than 3%) and ground reactivity with nitrate-bearing explosive emulsion. Keywords: coal, spontaneous combustion, carbonaceous shale, linear regression, premature detonation, iron ore mine.

Fire Hazards of Coal Storage Sites - Monitoring and Protection

Abstract Coal’s high propensity for spontaneous combustion can be the cause of fires in coal heaps located at mine sites, power plants, distribution or coal handling points. The application of appropriate measures to prevent coal from self-igniting and consequently to protect against fire. This paper presents optimal methods for monitoring and fire protection of coal stockpiles. The selection of suitable stockpiles for testing was carried out, followed by an assessment of the condition of the stockpiles (measurements of the temperature of the surface and interior of the facility, studies of the chemical composition of the atmosphere inside the facility, and the composition of the atmospheric air on the facility and in its immediate vicinity. Continuous monitoring of the dump was carried out for visible changes indicative of possible thermal phenomena. This was followed by the selection of the method and method of application of the antipyrogen (injection and spraying) and trials with the antipyrogen. Application of the product was followed by further monitoring of the dumps and analysis of the results obtained.

Study on the Law and Risk of Spontaneous Combustion of Residual Coal during Water Drainage in Goaf

Water–gas displacement occurring during the drainage of water-soaked goafs facilitates the oxidation of water-soaked coal. The characteristics of oxygen migration and the oxidation and spontaneous combustion (SC) of soaked residual coal during goaf drainage were explored through laboratory research, water drainage simulation and on-site measurement. The results reveal that compared with raw coal samples, the amount and rate of gas products of water-soaked coal samples are higher in the heating oxidation process, demonstrating a strengthened spontaneous combustion (SC) propensity. Its cross-point temperature falls and the apparent activation energy decreases by 1.43–8.75%, that is, the soaked coal sample is easier to spontaneously combust during the drainage of water-soaked goafs. Through simulation, it is found that after water is drained, air leakage in the goaf is significantly intensified, and the pressure difference inside and outside the goaf reaches 498 Pa. By taking the air inlet roadway as the air leakage point for fitting, it is found that the oxygen concentration in the air leakage range increases to 18% during water drainage. The simulation results are basically consistent with the on-site measurement. The on-site monitoring result shows that during water drainage of 7225 goaf in Qinan Coal Mine, water-immersed coal is more prone to spontaneous combustion, and air leakage leads to low-temperature oxidation of water-immersed coal, which increases the on-site temperature rapidly and increases the risk of spontaneous combustion in the goaf. With respect to water drainage in the goaf, an optimization measure of fixed-point and quantitative nitrogen injection during water drainage was put forward on site.

Study on Coal’s Spontaneous Combustion Propensity based on the Correlation between Oxygen Consumption and Heat Generation

ABSTRACT The fundamental reason for coal’s spontaneous combustion is that the amount of generated heat by oxidation exceeds that of heat dissipated. Therefore, the generated heat by oxidation and the amount of consumed oxygen are major indexes to evaluate coal’s spontaneous combustion propensity (CSCP). In this study, a method to simultaneously measure the consumed oxygen and generated heat was established by using constant temperature difference guiding method. On this basis, the relationship between the oxygen consumption and heat generation during low-temperature oxidation of four types of coal from six mines was studied. It was found that this method can quickly and efficiently obtain oxygen consumption and heat generation of coal during the low-temperature oxidation. The results reveal that the oxygen consumption of all the test samples are positively and linearly correlated with their heat generation. On this basis, with the aid of the oxygen consumption (to replace the heat generation), a theoretical basis for judging CSCP was obtained, which supports the identification of CSCP based on oxygen consumption.

Study on Pore Structure Change Characteristics of Water-Immersed and Air-Dried Coal Based on SEM-BET

ABSTRACT The pore structure of coal affects spontaneous combustion to a certain extent. The pore structure of the coal body will be altered after prolonged waterlogging and is highly susceptible to spontaneous combustion in the presence of elevated ground temperatures. To study the pore structure changes in the coal body after immersion and drying. The coal samples from the Jinjitan coal mine in Yulin, Shaanxi Province, the Tangkou coal mine in Jining, Shandong Province, and the Gaojiapu coal mine in Shaanxi Province were scanned by electron microscopy and low-temperature nitrogen adsorption experiments. This study indicates that after water immersion, the fracture pores of the coal body are developed, the morphology changes, and the percentage of pore fracture area increases significantly. With the increase in immersion time, the pore types of Coal samples A and C did not change, being delicate necked bottle-shaped pores and open, permeable pores, respectively. Coal sample B pore type developed from impermeable pores to open pores. The cumulative pore volume of coals from all three areas increased for the same pore size. The contribution of micropores to the BET specific surface area and total BJH pore volume was more significant than the original coal samples. The specific surface area of A90, B30, and C60 was larger than that of the other coal samples for the soaking time, so it can be assumed that A90, B30, and C60 have a higher propensity for spontaneous combustion. Underwater immersion, the pore type of coal samples from coal mines in Northwest China did not change, but the pore types of coal samples from coal mines in East China changed. Therefore, the pore structure of gas coal was easily changed underwater dissolution.

Effects of intrinsic properties, particle size and specific surface area on WOP and spontaneous combustion susceptibility of coal

This study examines the effects of intrinsic properties, particle size, and specific surface area (SSA) of coal on spontaneous combustion. Moreover, it analyzes the underlying mechanism of spontaneous combustion from particle size, SSA and gas adsorption perspectives. The susceptibility of coal to spontaneous combustion is studied by wet-oxidation potential (WOP) method. Coal residues left after the WOP experiment are analyzed for change in structural and compositional behaviour. The study revealed an inverse relationship between the particle size and SSA of coal. Spontaneous combustion susceptibility increased with increase in volatile matter and decreased with increase in fixed carbon content of coal. The optimum moisture content of coal resulting maximum spontaneous combustion tendency is determined 6%. Decrease in particle size and increase in SSA augmented the spontaneous combustion propensity of coal. With decrease in coal particle size from 425 to 850 to < 38 µm, the mean D50 value decreased by 27 times, the mean SSA increased by 42 times and coal spontaneous combustion susceptibility increased almost by 2 times. Critical coal particle size is determined 38–74 µm (D50 = 50.56 µm), below which no appreciable change in oxidation rate is observed. Moreover, two models are developed to predict the spontaneous combustion susceptibility of coal based on particle size (D50) and SSA.

A comparative kinetic study between TGA & DSC techniques using model-free and model-based analyses to assess spontaneous combustion propensity of Indian coals

Kinetic study of coal was carried out using simultaneous thermal analysis (STA) technique to assess the spontaneous combustion propensity of coal samples collected from various Indian coalfields having both fiery and non-fiery seams. The kinetic parameters were estimated by using both model-free and model-based analysis for both TGA & DSC data. The model-based method comprises four different consecutive reaction steps, viz. A→B→C→D→E for the spontaneous combustion process and the second reaction step (B→C) were used for this investigation. Chemometric analysis was applied to know the relation between the proximate analysis and activation energy of the samples using model-free and model-based techniques. The activation energy for the second reaction step of the model-based method for both TGA and DSC data showed a good relationship with the standard methods i.e., crossing point temperature (XPT) and Tgign of the samples. It indicates that the activation energy values at the oxidation stage (2nd stage) play a significant role in the spontaneous combustion propensity of coal. The study also reveals that the model-based analysis provided better results in comparison to model-free analysis to assess the spontaneous combustion propensity of coal.

Mechanistic Study on the Impact of the Occurrence of Various Basic Mineral Species on Spontaneous Combustion Propensity of Alkali‐Rich Low‐Rank Coal

AbstractSpontaneous combustion of low‐rank coal, especially alkali‐rich coal, is a key issue that determines the safety of storage and transportation. To understand the role of different forms of Alkali and alkaline earth metals (AAEMs) on coal spontaneous combustion, AAEMs in raw coal were sequentially eluted by water, ammonium acetate (NH4OAc) and hydrochloric acid (HCl) solution. The occurrence forms and mineralogical characteristics of AAEMs in raw coal and its derived coal samples were investigated. Further, the spontaneous combustion tendency of raw coal and derived coal samples was measured by programmed heating process and the chemical structure parameters were characterized by thermogravimetric analysis and differential scanning calorimetry (TGA‐DSC) and in situ infrared spectroscopy. The results show that demineralization mitigates the spontaneous combustion tendency by the increasing the ignition temperature, confirming the enhancement of inherent mineral on low‐temperature oxidation. In situ infrared spectroscopy results suggest that acid‐soluble and ion‐exchangeable AAEMs have greater catalytic impact on the oxidation reaction of coal than water‐soluble AAEMs. Specifically, the complete removal of AAEMs by acid‐washing treatment has the most obvious effect on the change of oxygen‐containing functional groups, which greatly reduces the generation rate of carboxylic acid, ester and anhydride groups during the low temperature oxidation. Moreover, calcium elements including ion‐exchangeable calcium and HCl‐soluble calcium can promote the oxidation capacity of coal at low‐temperature, which were verified by loaded coal samples.

TGA/DSC study to characterise and classify coal seams conforming to susceptibility towards spontaneous combustion

Thermogravimetric analysis/differential scanning calorimeter (TGA/DSC) technique along with basic coal characteristics study is carried out for eighty coal samples of Indian coalfields, to determine spontaneous combustion propensity behaviour of coal. TGA study of coal samples indicates that there is an increase in the mass of coal samples in the temperature range 150–350 ℃, which may be due to oxygen adsorption and absorption. The correlation and principal component analysis states that the component of proximate analysis (Mad, VMd, FR, and VR) have an acceptable correlation with the TGA experiments results i.e., Tgsh and Tgign. Multiple fixed nonlinear regression analysis shows that thermogravimetry (TG) experiment results Tgign may be the best index to categorise/classify the coal as per their susceptibility towards spontaneous combustion. The authors proposed four groups of classification as per their propensity towards spontaneous combustion depending upon the moisture (Mad), volatile matter (VMd), and TG ignition temperature from differential thermogravimetric (DTG) curve (Tgign) using hierarchal clustering analysis. The coal samples of different seams from Indian coalfield may be classified into four different clusters, viz. very highly/extremely susceptible (Tgign < 260 ℃), highly susceptible (260 ℃ ≤ Tgign < 290 ℃), moderately susceptible (290 ℃ ≤ Tgign ≤ 320 ℃), and poorly susceptible (Tgign > 320 ℃). The field observations and TGA/DSC experiment results with the following statistical analysis substantiate a similar assessment.

Adiabatic spontaneous coal combustion period derived from the thermal effect of spontaneous combustion

We propose a method for measuring the thermal effect of spontaneous combustion and the adiabatic spontaneous combustion period of coal based on the coupled thermal effects of coal and oxygen. A physical similarity reference test system was established by combining the characteristics of the thermal analyses. Using this system, coal samples from the main coal seam of the Zhao Xian mine, which has high propensity for spontaneous combustion, were tested and analyzed. The adiabatic spontaneous combustion period for coal was determined by curve fitting. Based on the heat equation, the adiabatic spontaneous combustion period for coal from the Zhao Xian coal mine was determined to be 95.9 h. Mutual verification among the adiabatic spontaneous combustion periods of coal was achieved through the same fitting procedure. The error between the two experimental results was less than 0.22 % with 0.01 variance, which proves that the experimental approach is applicable for calculating the adiabatic spontaneous combustion period of the Zhao Xian coal sample. Our results should facilitate theoretical and experimental evaluations of the likelihood of spontaneous coal combustion and its associated risks.

Compound effects of water immersion and pyritic sulfur on the microstructure and spontaneous combustion of non-caking coal

In order to reveal the compound effects of water immersion and pyrite on the microstructure and spontaneous combustion characteristics of non-caking coal, three coal samples from two neighboring mines were selected and prepared. The SEM observation, CO2 and N2 adsorption, XRD, XRF, FTIR tests and low temperature oxidation experiment were carried out on raw coal and soaked coal (S30). Showing that a large number of pores and fissures developed after waterlogging, especially M2 sample, the oxidation of its pyrite content (FeS2) leads to the promotion of pore development in coal matrix. Simultaneously, the presence of pyrite was more favorable for the generation of various functional groups and the maintenance of long chain aliphatic hydrocarbons during soaking. In conclusion, the presence of pyrite contributes to the micro-physical and chemical changes in the coal body during the water immersion, which further promotes the propensity for spontaneous combustion (combining data on oxygen consumption, gas products and CPT changes) of soaked coal.

Effect of feedstock, impurities, and rancidity on likelihood of spontaneous combustion of oil and biodiesel

Spontaneous combustion of biodiesel has been attributed to the destruction of multiple biodiesel production facilities. This paper investigates common conditions that affect the biodiesel and feedstock oil’s propensity for spontaneous combustion. The objective of this research was to develop a short-term test to determine the likelihood of developing spontaneous combustion in a long term. This study used a completely randomized block design which consisted of four main factors; two feedstocks (linseed oil and waste vegetable oil); two fuel types (raw oil and biodiesel); with or without impurity (sodium methylate, a catalyst used in biodiesel production, was used as an impurity); and two levels of rancidity (fresh and rancid fuel) with a total of 16 combinations. The tests prove all four factors play a significant role in affecting the likelihood of spontaneous combustion. A mathematical quantification was deemed necessary to identify the propensity of spontaneous combustion. A Spontaneous Combustion Severity Index (SCSI), a unitless number, was formulated, based on temperature rise normalized by experiment temperature, which correlates with heat generation patterns inside the reactors from 24-h experiments. Spontaneous combustion of biodiesel was observed under accelerated conditions using a fuel mixture with a SCSI number over 0.015.

Study on Intrinsic Relationship between Coal Mass Change and Heat Evolution during Different Stages of Coal Spontaneous Combustion

ABSTRACT A good understanding of the intrinsic relationship between heat evolution and mass change induced by coal–oxygen oxidation reaction is critical for accurately predicting the behaviour of spontaneous combustion. In this study, thermal subtraction method and six sets of characteristic temperatures were introduced to explore the intrinsic reaction between coal and oxygen which provides the main heat source responsible for coal self-heating. The heat evolution and mass change during coal spontaneous combustion were measured in real time using a sensitive differential scanning calorimeter (DSC) and a thermogravimetric analyzer (TG), respectively. The DSC and TG subtraction curves obtained from subtracting the results of nitrogen atmosphere from those of air atmosphere can reveal the heat change and mass change due only to the reaction between coal and oxygen. The term of q/m defined as the amount of heat release by the change of per unit of coal mass was introduced to study the intrinsic relationship between heat evolution and mass change during coal spontaneous combustion. The q/m values of the three coals show different evolution trends as coal temperature increases and are related to the mechanism of coal spontaneous combustion. The kinetics analysis of this intrinsic relationship was also carried out based on experimental data. Moreover, q/m can be used in the evaluation of spontaneous combustion propensity with respect to different coal types.

Coal spontaneous combustion characteristics based on constant temperature difference guidance method

Safety and environmental problems caused by coal mining and utilization are becoming increasingly serious. Studies in the coal spontaneous combustion characteristics based on constant temperature difference guidance method, which is of great significance for mine safety prevention and environmental protection. In this paper, a new method, Constant Temperature Difference Guidance Method, was developed in this paper for studying coal spontaneous combustion characteristics. By creating a constant temperature difference between coal temperature and ambient temperature, self-heating of coal is guided in this new method, which largely improves the efficiency and success rate of coal spontaneous combustion testing. Through analysis, this method was proved capable of providing a comprehensive description of coal’s low-temperature oxidation characteristics. Besides, through changing different values of the temperature difference, various factors of low-temperature coal oxidation under adiabatic environment can be calculated. Additionally, spontaneous combustion point and ignition point can be obtained by analyzing time-temperature curve of coal oxidation. These two points along with the duration to achieve these two points were used in this paper to develop a risk index for evaluating coal spontaneous combustion propensity.

Distribution of the functional groups in various coals with different spontaneous propensity

ABSTRACTThe distribution of oxygen-containing functional groups, aromatic hydrocarbons, and aliphatic groups in coal with different spontaneous combustion propensity were studied. Adiabatic method was used to determine the spontaneous combustion propensity of six different coal samples. FTIR was used to collect the spectrum of the coal, and the peak areas of the functional groups were analyzed. The proportion of C = C, CH3/CH2, OH, C = O, COOH, C-O, and C-O-O obtained based on the peak areas was calculated, and the spontaneous combustion propensity was characterized by the distribution of the functional groups. It was found that there is a good corresponding relationship between the content of aromatic functional groups and oxygen-containing functional groups with the self-heating time of the coal samples. The coal with shorter self-heating time has less aromatic groups and more oxygen-containing functional groups. With the increase of the spontaneous combustion propensity of coal, there are less aliphatic hydrocarbons functional groups generally. But among different coal samples, the contents of aliphatic hydrocarbons functional groups differ not great.

Evaluation effect of macerals petrographic and pyrite contents on spontaneous coal combustion in Tabas Parvadeh and Eastern Alborz coal mines in Iran

ABSTRACT Among the fossil fuels, coal is the most widely used one all over the world for different purposes and is a stable source of energy. Coal mining has serious hazards such as spontaneous combustion. Many factors can influence the tendency for occurrence of this phenomenon in coal mines and coal storages, one of which is the petrographic characteristics (maceral and pyrite contents). The petrographic characteristics are directly affected by the growth of coalification and origin of coal forming. In this research work, firstly, the maceral and pyrite contents in coal samples were identified in two various mines in Iran (Tabas Parvadeh and Eastern Alborz coal mines). Then the spontaneous coal combustion propensity of each sample was measured using the Crossing Point Temperature (CPT) test method. The aim for carrying out this research work was to measure the effect of the maceral and pyrite contents on the grade of spontaneous coal combustion propensity. It was found that the pyrite contents and the vitrinite, liptinite, and inertinite mean levels in the Tabas Parvadeh and Eastern Alborz coal mines were (2.08%, 53.30%, 9.91%, 34.71%) and (1.14%, 53.06%, 6.39%, 39.41%), respectively. The mean CPT value was 148.43ºC in the Tabas Parvadeh coal mines and 175.86ºC in the Eastern Alborz coal mines. By examining the results of the experiments and comparing the CPT values, it was found that with an increase in the liptinite and pyrite contents and a decrease in the inertinite content, the Tabas Parvadeh coal mines tend to have more tendency for spontaneous coal combustion in comparison with the Eastern Alborz coal mines.