Kinetics Of Coal Oxidation Research Articles

Study on Variation Law of Hazardous Areas in Goaf Based on Coal Oxidation Kinetics Theory

ABSTRACT Regarding various problems on safety production in coal mines, residual coal in goaf’s spontaneous combustion is extensive, determining its hazardous areas is critical to extinguish or prevent fires in goaf. Taking production practices at the 010803-working face of Wang Wa Coal Mine as background, a combined approach involving on-site experiments, laboratory tests, and numerical simulations is adopted to investigate “three-zone” spontaneous combustion as well as the distribution patterns of temperature fields in shallowly buried coal seams. The mathematical and physical models of spontaneous combustion in goaf are constructed. Upon oxidation of coal at low temperatures, experimental data about the intensity of release of heat and the rate of consumption of oxygen inform the construction of a kinetic model for coal oxidation in goaf. Utilizing field-measured data on air leakage, a distribution model for permeability within the working face’s goaf is constructed. To study combustion “three-zone” and temperature field distribution patterns in goaf under various air supply volumes, as well as to quantitatively analyze the relationship between the width and area of oxidation zones, the area of high-temperature areas, and the distribution of high-temperature sites in the goaf, with relation to the working face’s supply volumes of air. The results show that the maximum simulated air leakage in the goaf of the working face is 148.03 m3/min, with a relative error of just 1.4% compared to the measured value. Furthermore, validation confirms that the simulated oxygen concentration variation pattern closely aligns with the measured oxygen concentration. As the airflow rate at the working face increases, the oxidation zone width at the intake side gradually grows. While on the return side, it gradually decreases, resulting in a maximum width difference of 22.6 m. The goaf area’s concentration of oxygen exhibits an “S” shape distribution pattern. The overall temperature change range within the high-temperature zone of the goaf is 4.5 K, gradually migrating toward the deeper areas. As the air supply rate rises, oxidation zone area gradually decreases, exhibiting an overall sinusoidal distribution. Concurrently, the high-temperature zone area progressively expands, conforming to a Boltzmann function distribution. When the Air supply rate is 800 m3/min to 1120 m3/min, the areas of the two are linearly correlated with the air supply volumes. Under varying air supply volumes, the position of the high-temperature point within the goaf changes in both strike and dip directions, and the relative distance with the intake air corner follows an exponential function in both strike and dip directions. Through comprehensive analysis, it is concluded that the actual air supply volume on-site should ideally range between 800 m3/min and 1120 m3/min. This range effectively prevents and controls the hazardous area of spontaneous combustion in the goaf and optimizes the ventilation scheme.

Experimental study on the oxidation kinetics of coal in typical coal mining areas of the Southern Junggar coalfield, Xinjiang, China

Coal spontaneous combustion (CSC) is a disaster associated with coal mining that leads to loss of coal resources and environmental and human health issues. To investigate kinetic characteristics for oxidation of coal, three coal samples were collected from different coal mining areas in the Southern Junggar coalfield. Subsequently, the collected coal samples were ground into different particle sizes and tested using microscopic and macroscopic methods, including thermal gravimetric analysis, Fourier transform infrared spectroscopy, X-ray diffraction, and temperature-programmed oxidation. The results obtained are as follows: the sharpest absorption peak (002) indicates that graphitization is high. Furthermore, the results show that the SKS coal sample is prone to spontaneous combustion; the greater the aromatic hydrocarbon content is, the more difficult it is for CSC to occur, while the opposite is true for oxygen-containing functional groups. The SKS data confirmed this conclusion; the rate for generation of CO and CO2 controlled the possibility of SKS oxidation at 110 °C and provided an indication of the temperature. During the dehydration stage, the WD sample had the lowest activation energy, indicating that it was most susceptible to spontaneous combustion. During the combustion stage, the lowest activation energy was found for the SKS sample with particle sizes < 0.075 mm, indicating that particle size was one of the factors affecting spontaneous combustion. The activation energy for dehydration was significantly lower than that for combustion, which showed that the coal oxygen reaction was more likely to occur in the dehydration stage. Based on DSC curves, the SKS sample had the largest exothermicity, indicating that it would ignite more readily.

Coal oxidation characteristics and index gases of spontaneous combustion during the heating and cooling processes

In order to explore the changes during the coal cooling process in a closed fire zone (CFZ), the oxidation characteristics of the heating and cooling processes were thoroughly examined and compared. The heating and cooling processes under various oxygen concentrations of three coals were simulated with temperature-programmed experiment and coal spontaneous combustion experiment. Furthermore, the hottest spot, oxygen consumption, index gases, and oxidation kinetics between the gaseous production rate (f) and coal temperature (T), air supply (Q), oxygen concentration (CO2), and apparent activation energy (Ea) were analysed. The results revealed that the movements of the hottest spots did not overlap in the heating and cooling processes. Moreover, the oxygen consumption during the cooling process was higher than that during the heating process, and a lower oxygen concentration was associated with higher oxygen consumption, a condition in which coal strongly captured oxygen actively. The index gases during the heating and cooling processes notably differed, which were related to coal types, and during the forced heating process, the CO2/CO ratios of 1/3 coking coal and anthracite were higher than that of the forced cooling process, even under the oxygen-lean conditions, the CH4/C2H4 ratio of lignite during the forced cooling process was also notable. Moreover, the results in the cooling process of the temperature-programmed experiment in the carrier gases with 7% and 3% O2 were consistent with that of the spontaneous combustion experiment. Finally, the apparent activation energy at different stages of the heating and cooling processes was calculated, exhibiting inconformity, and apparent activation energy is lower in the cooling process, which is related to its higher oxygen consumption, and showing stronger oxidisability in the cooling process. In addition, through the results of coal oxidation kinetics, it is found that coal temperature affected the magnitude of apparent activation energy, the oxygen concentration impacted the oxidation activity of coal. These research results are valuable for informing the process of fire extinguishment and guiding the fire area unsealing in CFZ.

Philosophy of longwall goaf inertisation for coal self-heating control, proactive or reactive?

Self-heating of coal mass has been a long-standing thermal dynamic hazard and also an environmental concern during coal mining, processing, and storage. Especially self-heating of coal in longwall goaf poses a great danger to underground working crew and goaf inertisation is often practiced to combat the hazard. To investigate goaf inertisation with more insights regarding two different philosophies (i.e. proactive and reactive), a three-dimensional non-equilibrium thermal CFD model was developed to study heating evolution and a range of inertisation plans in a modern longwall. The theoretical model mainly incorporated a set of governing equations (e.g. energy conservation, continuity, and momentum balance) and coal oxidation kinetics which are determined by an adiabatic coal heating test. A comprehensive inertisation plan is developed. At initial stage of mining, a proactive inertisation plan is recommended because a heating in start-up area can develop to a fire incident if precautions are not implemented. While a reactive inertisation is suggested after longwall advancing a certain distance as the heating behind longwall face can be left in the fully suffocated zone before escalating to an advanced stage.

Proactive inertisation in longwall goaf for coal spontaneous combustion control-A CFD approach

Spontaneous combustion of coal has long been a thermal dynamic hazard during coal mining, storage, and transport, posing a great threat to coal mine safety. Especially coal spontaneous combustion in longwall goaf poses a great threat to underground working crew. To investigate such a mine safety issue with more insights and “what if” scenarios, this study employs CFD technique to develop a proactive inertisation plan in a longwall goaf. Based on real on-site conditions of the longwall goaf, a three-dimensional transient non-equilibrium thermal CFD model was developed to study heating evolution and proactive inertisation plans of the longwall goaf. The theoretical model incorporated a set of governing equations including low temperature kinetics of coal oxidation, energy and mass conservation, momentum balance, and continuity equation. After the base model (1000 m) was validated and calibrated with field gas monitoring data, another model (500 m model) was studied to obtain an optimum inertisation plan. Both steady state and transient simulations were conducted to study the flow dynamics of air velocity, oxygen ingress, dispersion of gaseous products and heating evolution in the longwall goaf. Based on the flow dynamics field, proactive inertisation plans using nitrogen to suppress the onset and development of goaf heatings were then developed and studied.

Reaction kinetics of coal oxidation at low temperatures

Adiabatic oven testing of coal to assess the propensity for self-heating produces data on the kinetics of the coal oxidation reaction. Repeat tests on the same coal at different starting temperatures ranging from 20 to 57 °C shows that there is a strong time–temperature effect on the reaction kinetics. At these low temperatures the initial reaction rate is predominantly non-Arrhenius, but converges to Arrhenius behaviour at temperatures above 70 °C. A new rate equation model has been derived that accounts for the change in kinetic behaviour in the low temperature region. This model is based on the concept of reactive site availability.

Kinetics Analysis of Heterogeneous Oxidation of Coal Particles in Supercritical Water

AbstractLittle research has been conducted on the heterogeneous kinetics of coal oxidation in supercritical water (SCW). Therefore, the oxidation of coal particles in SCW coupled with heat and mass transfer was investigated. The real coal particle temperature was estimated in terms of a simplified heat balance. With increasing particle temperature, the rate‐limiting step of the oxidation reaction gradually changed from the surface reaction to the mass transfer of oxygen. According to this result, the reaction rate model was built. The model could be used to predict the quantitative relationship among the particle size, the particle temperature, and the time to complete oxidation.

Oxidation of coal in industrial conditions. 1. Kinetics of natural oxidation

In the light of research on the artificial oxidation of coal in the laboratory, earlier results on the natural oxidation of coal in open storage at different coke Ukrainian plants are analyzed. On the basis of the kinetics of coal oxidation in natural conditions, the need to correct existing standards for open storage is evaluated. Since the dependence of the degree of oxidation do on the storage time is sigmoid, the results are analyzed on the basis of the Avrami-Erofeev equation for the kinetics of heterogeneous reactions. The constants of that equation and the rate constants of oxidation are calculated for enriched coal of different ranks (G, Zh, K, and OS) in summer and winter. The influence of the degree of metamorphism on the rate constant of natural oxidation in summer and winter is mathematically described. On the basis of the Avrami-Erofeev equation, limiting open-storage times for the coal are established. The earlier limiting storage times must be corrected.

Development of non-isothermal TGA–DSC for kinetics analysis of low temperature coal oxidation prior to ignition

The present work was aimed at developing a method for determining the kinetics of low temperature coal oxidation based on thermogravimetric analysis and differential scanning calorimetry (TGA–DSC). The analyses of TGA–DSC on non-isothermal oxidation of three coals at a single heating rate were carried out. The measured heat release rates and derived apparent kinetic parameters of low temperature oxidation were compared with those from the tests of basket heating methods (crossing point temperature and heat release method). The results demonstrated the equivalence of the TGA–DSC technique and the basket heating methods in measuring the heat release rate and kinetics of coal oxidation in the temperature range of about 100–150°C. Moreover, the TGA–DSC method can also measure the kinetic parameters of coal oxidation at the temperatures from ∼150°C to that of ignition occurring, which were shown to be different from those at the temperatures below ∼150°C. Therefore, the TGA–DSC method developed here is promising to be applied as a cost effective technique for deriving the kinetics of low temperature oxidation to describe coal spontaneous combustion process prior to ignition.

A Study of the Phenomenon of Mass Increased by Coal Oxidation Prior to 400 ℃: Governing Mechanism and the Associated Chemical Kinetics

The reaction mechanism and kinetics of coal oxidation at temperatures prior to 400 °C were studied theoretically, with a focus on the phenomenon of mass increase observed by thermo-gravimetric analysis (TGA). In light of the existing understanding on coal oxidation, chemical reactions taking place at moderate temperatures were simplified into three parallel but competitive reaction sequences, including water release, oxygen adsorption and decomposition of the oxygenated compounds, as well as the direct “burn-off” reaction. Using the non-linear least-square fitting technique, the kinetic parameters of all the reactions occurring within the temperature range studied were determined by fitting the sets of TG and DTG data collected at the oxygen concentrations of 10%, 21% and 40% in volume, respectively. The modeling results are in excellent agreement with the experimental data, confirming the reliability of the model established. It is proved that the mass increase during coal oxidation at moderate temperatures is primarily contributed by the oxygen adsorption and the subsequent decomposition reactions, while influence of the direct “burn-off” reaction on this phenomenon is relatively trivial. Since the mass increase phenomenon is a compromised result of the principal reactions involved, and the kinetic parameters of the oxygen adsorption reaction cannot be determined with accuracy based upon this phenomenon with a utilization of a single-reaction assumption. Enrichment of O2 in the reaction medium speeds up the rates of oxygen adsorption reaction and the decomposition reaction. However, we did not observe the significant effect of the oxygen concentration on the direct “burn-off” reaction, solely due to the nature of the experimental data selected No. 8 战 婧等:煤氧化增重现象中的控制反应及其动力学解析 981 and the techniques utilized for retrieving the kinetic data for the reactions involved.

Thermal analysis of kinetics of coal oxidation

Thermal analysis was used to study the oxidation and thermal degradation reaction of Yangquan coal. One method is that the coal was heated to a certain temperature and held at that temperature for 45 min; Another method is that coal was heated to a temperature and held for 45 min, then it was taken out and the new coal sample was re-put with other temperature's experiments. Two different experiments and their dynamic characterizes were compared, showing that the change degree of TG curve is different from each other. The process of coal oxygenolysis belongs to the first-order of chemical reaction. The activation energy increases along with the reaction process. Moreover, the oxidation of coal is a process that contains lots of reaction steps which mutually promote and connect.

Conversion and Kinetics of the Oxidation of Coal in Supercritical Water

The oxidation of coal in supercritical water was explored by using H2O2 as the oxidant source in a bench-scale semicontinuous installation. The conversion and kinetics of coal oxidation in supercritical water (SCW) medium were investigated. The reaction parameter effects on the conversion and kinetics of coal supercritical water oxidation (SCWO) were determined. Increasing temperature, H2O2 concentration, or the flow rate of H2O2 solution enhances the oxidation of coal in SCW. The oxidation of coal in SCW is a pseudo-first-order process.

Model for coal oxidation kinetics. 2. An effectiveness factor interpretation

Rate expressions are developed to describe the oxidation of coal in a fixed-bed reactor under conditions of significant diffusional effects. The model defines individual effectiveness factors for carbonic gas formation, oxygen deposition, and water production, to modify a chemical model derived previously. Experiments carried out on seven coals ranging in rank from lignite to bituminous to anthracite. All the coals exhibited particle size effects and the results fit the model in most cases to within ±10% both in the rate of overall oxidation and with respect to the individual reaction rates for several products. Activation energies and effectiveness factors were obtained for each coal and for each reaction. The effluent gas CO 2 CO ratio was found to be virtually constant for all five bituminous coals, being independent of reaction temperature, coal particle size, and oxygen partial pressure.

Model for coal oxidation kinetics. 1. Reaction under chemical control

Rate expressions are presented to describe the oxidation of coal in a fixed-bed reactor under mild conditions, i.e. conditions which are useful in pretreatment for gasification. A model for chemical control is developed based upon the concept of a triple-path coupled burn-off adsorption site. The model incorporates the effects of temperature, oxygen partial pressure, and coal surface area, and is the first to account for the product distribution of gaseous C0 2, CO, and H 2O, as well as oxygen deposited on the coal surfaces. The overall rate of oxidation is limited by the amount of reactive surface area in the coal particles and is assumed to be reduced as a result of the deposited oxygen which covers potentially reactive sites. This model differs significantly from the previously formulated model of Kam, A. Y. et al. Chem. Eng. Sci. 1976, 31, 815 which postulated external mass transfer control.

On the kinetics of coal oxidation

AbstractA series of seven coals of different ranks and from various locations were heated in air under relatively mild conditions to measure the rates of oxidation and the production of carbonic gases in the effluent stream. The gas flow rate, coal particle size, and reactor temperature were changed as independent variables. Each sample was exposed for from 6 to 9 hr under atmospheric pressure at temperatures in the range of 200° to 250°C. The results show two kinds of rate behavior depending primarily on the relative porosity of the coal under study. The small pore coals followed the expectations of the earlier Kam‐Hixson‐Perlmutter (KHP) model, but the large pore coals gave rates sensitive to transport effects. Correlations were also obtained on the ratio of carbon dioxide and carbon monoxide produced and on the relationship between the carbon content of an exposed coal and its heating value.

The kinetics of coal oxidation

Seven coals from across Canada were oxidized, and a kinetic analysis made from the data obtained. It was established that the equation 1, ▪ is a valid rate of sorption of oxygen in the temperature range 110 to 170°C.

Some aspects of the kinetics of oxidation of coal I. The correlation of coal oxidation kinetics

AbstractThe results of the oxidation of an Australian coal have been successfully correlated by the Elovitch equation: The relationship between the parameters A and B of this equation and the mass of coal used in the experiment leads to the over‐all kinetic equation: