Low-temperature Oxidation Stage Research Articles

Thermodynamic Characteristics of Oxidation and Combustion of Coal under Lean-Oxygen Conditions

Coal spontaneous combustion (CSC) often occurs in environments that are poorly ventilated or under lean-oxygen environments in coal mines or coal seam outcrops. The understanding of the thermodynamic properties of CSC under lean-oxygen conditions is important to avoid safety and environmental problems. In this paper, the mass variation and critical temperature (the minimum temperature required for coal reaction in each stage) of six coals during CSC under lean-oxygen conditions were investigated using thermogravimetric methods. Furthermore, the thermodynamics parameters and kinetic compensation processes during CSC under lean-oxygen conditions were analyzed. The results showed that the oxidation and combustion of coal under lean-oxygen conditions was affected by the coal rank and oxygen concentration. The increase in the critical temperature was more significant when the oxygen concentration was reduced from 10 to 5%. The mechanism functions most likely to cause CSC at different oxygen concentrations were similar. The reaction mechanisms of high-rank coals at a low-temperature oxidation stage were more influenced by oxygen concentration than low-rank coals. Evaluations of kinetic behavior showed that activation energy decreased in a linear manner as the oxygen concentration decreased. The mathematical relationship between the activation energy and the pre-exponential factor indicated that there was a kinetic compensation process during CSC under lean-oxygen conditions. It is also worth noting that the effect of the coal rank on the thermodynamic characteristics of CSC is better than that of oxygen concentration. This work is helpful for enhancing the prediction and prevention of CSC.

Study on the Effect and Mechanism of Water Immersion on the Characteristic Temperature during Coal Low-Temperature Oxidation

Spontaneous combustion of dried water-immersed coal (DWIC) wastes many coal resources, and so the effect of water immersion on the characteristic temperature during coal low-temperature oxidation and its possible reasons were investigated. The characteristic temperatures of coal oxidation were obtained by thermogravimetry. The possible causes for the changes in characteristic temperatures were analyzed from the pore structure and microscopic function groups by means of liquid nitrogen adsorption/desorption experiment and in-situ Fourier transform infrared spectroscopy. The results show that, in the early stages of coal low-temperature oxidation, the characteristic temperatures of DWIC samples were lower compared to those of raw coal samples. The differences between the characteristic temperatures of the DWIC and raw coal first increased but then decreased as the temperature rose. When the coal’s temperature rose to the maximum weight gain temperature, the differences had almost disappeared. Water-immersion reduced the coal’s specific surface area but increased the average diameter of its pores and the total pore volume. These changes in the coal pores were beneficial for oxygen migration and transport. During the whole low-temperature oxidation stage, the DWIC hydroxyl concentration is higher compared to that of raw coal, and this made the initial reaction of coal and oxygen more likely to occur. In the late low-temperature oxidation stages, the methyl concentration had a considerable influence on the coal–oxygen reaction.

Application of terahertz dielectric constant spectroscopy for discrimination of oxidized coal and unoxidized coal by machine learning algorithms

Coal spontaneous combustion has always been a global catastrophe, which threatens valuable coal resources, human safety and health, and the environment. The key to preventing coal spontaneous combustion is early monitoring at the low-temperature oxidation stage of coal. The terahertz dielectric property measurement system and machine learning algorithms were used to test the feasibility of a non-contact real-time fast discrimination of oxidized coal from its unoxidized counterparts. Logistic regression analysis (LRA), Support vector machines (SVM), and random forest (RF) models were applied to classify coals based on oxidation degree. The results proved that differences of dielectric properties at terahertz band between unoxidized and oxidized coals did exist. The best classification result with an accuracy of 87.50% in the prediction set was achieved using the SVM method based on Gaussian kernel function combined with low-frequency (75-110 GHz) terahertz spectrum of the imaginary part of dielectric constant. The results prove that terahertz dielectric constant spectrum, combined with machine learning algorithms, would be a promising technique to monitor coal spontaneous combustion with high efficiency.

The evolution law of functional groups during the heating of coal in oxygen and oxygen‐free atmospheres

AbstractThere is a change in both the quantity and type of functional groups due to the fact that they become oxidized and pyrolyzed during the spontaneous coal combustion process. In order to study the oxidation and pyrolysis mechanisms that are at play during the evolution of these functional groups, in situ Fourier transform infrared (FTIR) spectroscopy was employed to test the real‐time evolution of these functional groups in both air and argon atmospheres. The differences found between functional groups when under an oxygen oxygen‐free environment were analyzed. The results show that when the temperature exceeds 200 °C, the changes in functional groups steadily increase and that regardless of whether the atmosphere is made up of argon or air, the oxygen‐containing functional groups change significantly. In this situation, −OH, −CH3, and −CH2‐ are important reactants in the whole heating process; −COOH and −C=O are reactants during the low temperature oxidation stage, but when reaching the high temperature stage, they become the products; C‐O‐C and C=C is a stable reaction product. During the coal spontaneous combustion process, oxidation and pyrolysis work as two coupling reactions: The heat released by the oxidation reaction pyrolyzes the macromolecular structure and coal‐oxygen complexes, while a large number of active groups are produced during pyrolysis, which then provides the reactants for the oxidation reaction. These results reveal the changes that functional groups go through during the process of oxidation and pyrolysis, and provide a theoretical basis for the development of efficient fire‐fighting materials. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

Experimental Study on the Inhibition Effects of Nitrogen and Carbon Dioxide on Coal Spontaneous Combustion

Inert gases can effectively inhibit coal spontaneous combustion. In this paper, the inhibition effect of inert gases (N2 and CO2) on coal spontaneous combustion was studied. In the low-temperature oxidation stage, the constant-temperature heat release and apparent activation energy of coal sample were measured and calculated by the C80 micro-calorimeter. In the high-temperature combustion stage, the critical temperature, maximum peak temperature, ignition temperature, and burn-out temperature of coal samples were analyzed by the synchronous thermal analyzer. The results demonstrate that with the decrease of O2 concentration, the oxidation heat release of coal samples drops gradually while the apparent activation energy increases gradually. In the N2 and CO2 atmospheres, as the O2 concentration is reduced to 1.5% and 3%, respectively, the value of apparent activation energy changes from negative to positive, and the spontaneous reaction transits to a nonspontaneous reaction. The TG-DTG (thermogravimetric-derivative thermogravimetric) curve of coal sample in the high-temperature combustion stage indicates that the critical temperature exhibits a W-shaped trend with the decrease of O2 concentration, which also leads to gradual increases of maximum peak temperature, ignition temperature, and burn-out temperature. The above results signify that increasing the inert gas concentration can gradually reduce the oxidation and combustion rate and improve the inhibition effect on coal spontaneous combustion. In addition, when the O2 concentration is the same, the inhibition effect of CO2 on coal spontaneous combustion is superior to that of N2.

Experimental study on dynamic characteristics of low temperature oxidation for Bohai offshore light oil under reservoir conditions

Air flooding has shown to be a promising method for enhanced oil recovery (EOR). The air injection into reservoir causes complex oxidation reaction, resulting in the change of the properties and components of crude oil. Most studies on thermal behavior and kinetics of crude oil were not conducted under realistic reservoir conditions, bringing difficulties to understand the suitability of air flooding for a light oil reservoir. The objectives of this research were to investigate dynamic characteristics of low temperature oxidation (LTO) of Bohai offshore light oil under realistic reservoir conditions. Thermogravimetry experiments (TG), static and dynamic oxidation experiments were used to evaluate the oxidation reaction of Bohai light oil reservoir. Besides, the thermal behavior of the light oil and alteration of oil components during LTO were comprehensively measured. The results indicated that there were three stages for light oil in LTO stage, the first and second stages of LTO showed positive activation energy but the third stage showed negative activation energy, manifesting the LTO stage might include multiple mechanisms. The LTO of light oil was promoted by pressure, and the negative temperature coefficient (NTC) stage of LTO might be controlled by medium oil component (C7-C17). Additionally, oxygen consume rate of dynamic oxidation experiment was much higher than that of static oxidation experiment, which was closer to the realistic reservoir conditions. This study can provide insights to the LTO of light oil reservoir and explore the potential of air flooding based on EOR projects.

Thermokinetic Characteristics of Jurassic Coal Spontaneous Combustion Based on Thermogravimetric Analysis

ABSTRACT To prevent coal fire disaster, a precision grading warning method was previously proposed for coal seams that are susceptible to spontaneous combustion. The uncertainty in the reaction mechanism of the spontaneous combustion of coal greatly restrains the in-depth understanding of the fine division theory in the course of coal combustion. In this paper, further research has been conducted to study the mechanism of this method. In this study, thermogravimetric analysis was carried out on the Jurassic coal of three different metamorphic grades. The variations of the TG/DTG curves of the coal samples at four heating rates were compared to examine their kinetic model and kinetic compensation effect. For the three samples of the Jurassic coal, a similar trend was obtained for the TG and DTG curves at the different heating rates. With an increase in the heating rate, the hysteresis phenomenon appeared in the TG/DTG curves. In the low-temperature coal oxidation stage, the most probable mechanism function of Coal A and Coal B was obtained to be No.9 (G(α) = [1-(1-α)1/2]2, f(α) = (1-α)1/2[1-(1-α)1/2]−1), and that of Coal C was No.14 (G(α) = [-ln(1-α)]2, f(α) = (1-α)[-ln(1-α)]−1/2). A significant kinetic compensation effect was also observed in the oxidation stage. Despite the difference in the metamorphic grade, the reaction process of the low-temperature oxidation, as well as the rate-determining step was the same for the three samples.

Mechanistic and kinetic insight into catalytic oxidation process of heavy oil in in-situ combustion process using copper (Ⅱ) stearate as oil soluble catalyst

In this study, copper (Ⅱ) stearate was proposed as oil-soluble catalysts for catalyzing heavy oil oxidation in in-situ combustion (ISC) process to improve the efficiency of ISC for heavy oil recovery. Its catalytic mechanism and kinetics were deeply investigated by joint use of TG-FTIR, autoclave experiments, FESEM-EDX, and XPS, etc., together with isoconversional kinetic methods. We find that the addition of copper (Ⅱ) stearate initiated both efficient homogenous and heterogenous catalytic oxidation/combustion process of heavy oil. In low-temperature range, copper (Ⅱ) stearate (before its full decomposition) played a homogenous catalytic role in low temperature oxidation (LTO), and in high-temperature range, in-situ formed CuO nanoparticles (after the full decomposition of copper (Ⅱ) stearate) played a heterogenous catalytic role in the formation and combustion process of fuel (coke-like residues) in fuel deposition (FD) and high temperature oxidation (HTO) stages. Specifically, the addition of copper (Ⅱ) stearate significantly reduced the values of Eα of all reaction stages (LTO, FD, and HTO), especially at the later stage of LTO, FD and the beginning of HTO (the maximum values of Eα were decreased from about 500–600 KJ/mol to 300–400 KJ/mol), decreased the energy required to overcome reaction barriers, and improved the formation rate and quality of coke-like residues, which thus promotes the formation of coke-like residues and their combustion a more continuous process. Such a superior catalytic effect makes copper (Ⅱ) stearate have a great potential in improving efficiency of ISC process for heavy oil recovery.

Isokinetic analysis on the oxidation of Jurassic coal: a case study of samples from Xinjiang, China

Jurassic coal samples from Liuhuanggou (LHG) and Wudong (WD) coal mines, Xinjiang Province, China, were selected for oxidative thermal energy experiments. A temperature-programmed experimental system measured the variation of gaseous products and the changes in thermal energy release with temperature. Thermokinetic parameters, namely the apparent activation energy (Ea) and pre-exponential factor (A), of the coal samples for four heating rates were determined using a thermogravimetric analyser. During the low-temperature oxidation stage, two samples had temperature curves characterised by their production levels of the index gas (CO); the temperature changes were approximately equal, but the critical temperatures differed. The WD sample produced less CO gas. However, the maximum and minimum exothermic strengths of the two samples had some similarities to the temperature change curves. The characteristic temperatures for the coal samples were discovered using different heating rates. For the WD sample, the characteristic temperature varied according to the heating rate, and the thermogravimetry (TG) and derivative TG curves of the samples lagged with an increase in the heating rate. The characteristic temperatures of the sample increased with the increase in the heating rate. Isokinetic analysis (the Flynn–Wall–Ozawa and the Kissinger–Akahira–Sunose models) was used to calculate the thermokinetic parameters of the sample. The range of the heating rates was 2 to 15 °C min−1 for oxygen concentration of 21 vol%, and the Ea of the samples decreased with the increase in the heating rate.

Combustion Characteristics of Coal Dust Containing Oil in the Oxygen-enriched Environment

ABSTRACT An oily coal dust that is difficult to remove is easily formed on the surface of coal mining equipment. However, high-power equipment generates a higher surface temperature, which can promote spontaneous combustion of coal, especially in the oxygen-enriched environment of the refuge chamber. This study investigated the effects of lubricating oil on coal spontaneous combustion under different oxygen concentrations using a temperature-programmed experiment, Fourier transform infrared spectroscopy and thermogravimetry-differential scanning calorimetry. The results show that the activation energy is inversely proportional to the lubricating oil content during the low-temperature oxidation stage. With a mass ratio of coal to lubricating oil of 2:1, compared with raw coal, the activation energy was reduced by 6.46 KJ/mol. With a higher lubricating oil content of coal and an increased oxygen concentration, the absorption peak intensities of -CH3, -CH2, -OH were lower and the C-O absorption peak intensity was higher. Therefore, lubricating oil promotes coal spontaneous combustion and the promoting effect is proportional to the lubricating oil content and oxygen concentration. Furthermore, the difference in heat released indicates that the effect of lubricating oil on coal spontaneous combustion is the result of the interaction between coal oxidation and lubricating oil oxidation.

Effects of aromatics, resins, and asphaltenes on oxidation behavior and kinetics of heavy crude oil

This research is intended to investigate the effects of aromatics, resins, and asphaltenes on the oxidation behavior of heavy crude oil. The results showed that aromatics had inhibition effects on exothermic behavior of the oil. The thermal effect of oil + resins in the low temperature oxidation (LTO) stage was more intense relative to other mixtures. Compared to other samples, the LTO activation energy of the oil plus asphaltenes was the lowest, 32.28 kJ/mol, indicating that asphaltenes obviously reduced the difficulty of LTO reactions of the oil. Aromatics, resins, and asphaltenes might play a negative role in the reactivity of coke formed.

Study on the Change of Dielectric Properties and Chemical-Mechanism during Coal Low-Temperature Oxidation

The study aims to investigate the variation of the coal dielectric properties during coal low-temperature oxidation and the effect of oxidation product on the coal dielectric properties. Four different types of coal were prepared under low-temperature oxidation condition, and the coal dielectric properties were measured with an impedance analyzer at frequencies ranging between 20 Hz and 30 MHz. The oxygen-containing functional groups in oxidized coal samples were semi-quantitatively evaluated using Fourier transform infrared spectroscopy. Low-temperature oxidation stage of coal spontaneous combustion could be predicted according to the change of coal dielectric properties in the process of temperature rise. It was found that the dielectric constant of coal with high water content decreased exponentially with temperature. For coal sample with low water content, the dielectric properties changed piecewise linearly with temperature. Coal dehydration was considered to be the reason for the decrease of the dielectric constant from 30 °C to 120 °C. The increase of the relative content of oxygen-containing functional groups, especially carbonyl compounds, could be the reason for the rise of the coal dielectric constant from T2 to T3.

Effect of the heating rate on the spontaneous combustion characteristics and exothermic phenomena of weakly caking coal at the low-temperature oxidation stage

The low-temperature oxidation stage of coal spontaneous combustion (CSC) is crucial due to its impact on likelihood of spontaneous fire, as well as fire prevention in coal storage and underground mine environments. A detailed understanding of the oxidation characteristics of coal can provide a systematic understanding of CSC. A temperature-programmed experiment was conducted to simulate CSC in its low-temperature oxidation stage under different heating rates. The characteristic temperatures and oxidation performance were calculated using related functions, and the temperature distribution in space was obtained using three different measuring points. An additional experiment using a C80 micro-calorimeter was also performed to examine the heat flows. The coal sample was subjected to different heating rates, and variations in endothermic and exothermic heats were recorded using the derivative of the heat flow concerning time (dHFCT). A low heating rate promoted the exothermic effects of coal oxidation, resulting in a more thorough exothermic process.

Analysis of Kinetic Characteristics of Merey Crude oil Pyrolysis by Isoconversion Method

As an important chemical reaction during in-situ combustion, pyrolysis reaction is related to the whole combustion process. To understand the kinetic characteristics of the pyrolysis reaction more clearly, three models, FWO, Friedman and KAS, were used to fit and calculate the thermogravimetric data of Merey crude oil by three linear heating rates under air atmosphere and analyze the kinetic parameters of crude oil at different conversions. The results show that KAS model provides the fitting result similar to the FWO model, but Friedman model provides smaller result, and the FWO model provides the highest linear correlation and the most accurate calculation results among the three models; the gasification stage has the lowest activation energy, followed by low temperature oxidation stage, the pyrolysis stage has the highest activation energy and the highest energy consumption and is the core reaction in the whole process, and the high temperature oxidation stage has a bit lower activation energy than the pyrolysis stage among the four stages of thermal reaction of Merey crude oil.

Real-time analysis of the changing trends of functional groups and corresponding gas generated law during coal spontaneous combustion

In the process of spontaneous combustion, coal samples undergo oxidation and pyrolysis reactions, and various gases are produced when functional groups transform with each other. This paper conducted in situ FTIR and high-temperature oxidation experiments on raw coal samples to investigate the changing trends of functional groups and corresponding gas generated law. Experimental results show that at the early stage of low-temperature oxidation (before 90 °C), CO production is the result of oxidation and decomposition reactions, and water evaporation leads to significant changes in OH. At the late stage of low-temperature oxidation (100–200 °C), CO increased exponentially with the increase of oxygen consumption. After 200 °C, the activity of each functional group increased significantly and the O2 supplied were completely consumed. Meanwhile, the thermal decomposition of aromatic hydrocarbons CC in coal produced large amounts of CH4. When the oxygen-nitrogen ratio reaches 0.15 or less, the nitrogen injection has a significant inhibitory effect on the late stage of the low-temperature oxidation process. However, the higher the temperature, the less obvious the inhibition effect of nitrogen on the activity of functional groups. The results are applicable to the study on the thermodynamic disaster warning, prevention and control.

Inhibition mechanism of spontaneous combustion by nano-sized complex inhibitor for Chinese lignite in low-temperature oxidation

In order to analyze the inhibition mechanism of inhibitor in the process of low-temperature oxidation, nanometer aluminum hydroxide [Al(OH)3], nano-sized magnesium hydroxide [Mg(OH)2], and mixtures of Al(OH)3 and Mg(OH)2 were investigated as inhibitor. The low-temperature oxidation process of blended inhibitor and lignite was analyzed using automatic heating system and oxidation of O2-TG. Our results showed that Al(OH)3 effectively delayed the water evaporation process of coal samples in the early stage of low-temperature oxidation. No strong increases of oxygen and weight had been observed in the stage of increasing oxygen gain when both Al(OH)3 and Mg(OH)2 were used as inhibitor. Content change of active groups such as hydroxyl group, carbonyl group, carboxyl group and ether oxygen group of samples in the process of low-temperature oxidation were quantified by both FTIR and XPS. And our results shed light on the inhibition mechanism of spontaneous combustion by inhibitors for Chinese lignite exist in low-temperature oxidation. Moreover, our results demonstrate that the performance of multi-component inhibitor is better than that of the single component inhibitor in low-temperature oxidation. Al(OH)3 and Mg(OH)2 prohibit the oxidation in the low-temperature region and high-temperature region, respectively. Therefore, the complex inhibitor can effectively decrease the low-temperature oxidation of coal, and can effectively prevent spontaneous combustion of coal.

Effect of calcite and dolomite on crude oil combustion characterized by TG-FTIR

This study is aimed at studying the influence of calcite and dolomite on crude oil combustion using TG-FTIR technique. According to the TG-DTG curves and the released gaseous products, the entire combustion process of crude oil can be divided into three regions: low temperature oxidation (LTO), fuel deposition (FD) and high temperature oxidation (HTO). The presence of calcite or dolomite significantly promoted the continuity of reactions, thus resulting in a smooth transition from LTO to FD, and further to HTO. Specifically, the explicit boundary between LTO and FD disappeared, the reaction of fuel formation started in the later stage of LTO, and the FD process was also partly merged with HTO reaction into one reaction region, even the simultaneous occurrence of FD and HTO reactions was observed for calcite. In general, calcite and dolomite have distinct catalytic effects on crude oil combustion. Their existence made fuel deposition and its combustion easier by significantly reducing the activation energy (mainly in FD stage and the beginning of HTO stage) from about 250 to 450 kJ/mol to 150–225 kJ/mol, which thus leads to a more continuous FD-HTO process. Calcite has a superior catalytic effect than dolomite as it achieved lower values of activation energy with smaller fluctuation (maximum 175 and 225 kJ/mol for calcite and dolomite, respectively). These results indicate that the presence of calcite and dolomite (especially calcite) is favourable for accelerating crude oil combustion reaction in in-situ combustion (ISC) process.

Research on the Characteristics of the Coal–Oxygen Reaction in a Lean-Oxygen Environment Caused by Methane

Coal mine fire often occurs in lean-oxygen environments caused by methane. In order to clarify the characteristics and mechanisms of coal-oxygen reaction under these conditions, the coal-oxygen reaction experiments were carried by using thermogravimetric analysis (TGA)-gas chromatograph (GC) combined technology in the range of 30-800°C. The thermogravimetric-subtraction/derivative thermogravimetric-subtraction (TG-S/DTG-S) curves which can more accurately reflect the coal-oxygen process were obtained by subtracting the TG/DTG curve of coal pyrolysis under the condition of pure N2. Through TG-S/DTG-S curves, the coal-oxygen reaction was divided into low temperature oxidation stage, accelerated oxidation stage and combustion stage. In the low temperature oxidation and accelerated oxidation stages, methane delayed the coal-oxygen reaction as an inert gas. Under the same oxygen concentration, the effect of methane on reducing CO, CO2 production and increasing ignition temperature (Ti) of coal was obvious than...

Oxidation kinetic evaluation of the low temperature oxidized products of Tahe heavy oil characterized by the distributed activation energy model

In view of the significance of low temperature oxidation (LTO) for in-situ combustion (ISC) of heavy crude oil and to provide further data for Tahe ISC numerical simulation, this research aimed to investigate the non-isothermal oxidation behavior of heavy oil after static LTO reactions. Thermogravimetric (TG) and differential scanning calorimetry (DSC) techniques were used to characterize oxidation behavior, and distributed activation energy method (DAEM) was used to calculate kinetic parameters. The results showed that the non-isothermal oxidation process of static LTO oxidized oil was the result of the interaction between residue and coke, and coke can significantly reduce the activation energy of oxidized oil. Only one HTO region was identified for the coke from DTG/DSC curves, and it had the lowest activation energy (114.57 kJ/mol) and the highest heat enthalpy (24.3 kJ/g), indicating high oxidative activity with maximum heat release potential. A satisfactory corresponding relation between DTG and DSC curves was presented for all samples in FD and HTO regions, which indicated that the sequential reaction mechanisms, fuel deposition and combustion, were undergone. In LTO region, there was a temperature difference between the peaks of the mass loss and heat flow, indicating that the LTO process was more complex with a multi-step control mechanism should be considered to analyze and simulate the LTO stage.

Thermal Behavior and Kinetic Triplets of Heavy Crude Oil and Its SARA Fractions during Combustion by High-Pressure Differential Scanning Calorimetry

In situ combustion (ISC) has been regarded as an efficient technique for the exploitation of heavy oil reserves. In this work, the thermal behavior of one heavy crude oil and the fractions of its saturates, aromatics, resins, and asphaltenes (SARA) during combustion was thoroughly investigated using high-pressure differential scanning calorimetry. Two typical isoconversional methods were adopted to determine the variation of activation energy (E) and frequency factor (Ar) versus conversion degree in the course of the reaction, followed by the evaluation of the reaction model, f(α), via the master plot method. The results indicated that the heavy oil encountered larger thermal release caused by low-temperature oxidation (LTO) reactions rather than high-temperature oxidation (HTO) reactions, suggesting that appreciable heat could be available within the low-temperature range. Saturates showed a notably apparent heat release in the LTO reactions. For aromatics, the exothermic effect at the LTO stage was appa...