Article

A review on numerical solutions to self-heating of coal stockpile: Mechanism, theoretical basis, and variable study

Authors:
  • China Coal Reseach Institute
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Abstract

Self-heating or even spontaneous combustion of stockpiled coal, which is likely to outbreak under favourable circumstances during its transport, process, and storage, is a long-standing thermal dynamic hazard. This hazard is harmful in diverse aspects: causing loss of coal resource and caking property, raising safety concerns upon occurrence of open fire, and giving off noxious/greenhouse effect gases. Due to the complexity of involved physical process (e.g. heat and mass transport) and chemical process (e.g. coal oxidation), formulating an analytical solution to the problem with or even without a transient approach would be a daunting task and the problem is thus more often addressed numerically. So far many numerical models to self-heating of coal have been developed and to summarise these erratic findings, this work critically reviewed theses numerical solutions since the last four decades. Mechanism of self-heating on coal mass and low temperature coal oxidation especially kinetic modelling of coal oxidation is firstly investigated to clarify the involved physical and chemical processes. On basis of the mechanistic understanding, theoretical derivations and progressive advances on governing equations like energy, mass, and momentum conservation are reviewed and compiled in details. Through parametric studies or sensitivity check these models produced fruitful but slightly inconsistent findings. Therefore to provide industry more unbiased and comprehensive guides, the present work examined the influences of various contributors including wind flow, stockpile dimensions, coal particle size, moisture content, and packing porosity on the self-heating behaviour of stockpiled coal. Last not the least, major challenges and perspectives this subject may have are briefly discussed.

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... Thus, this aging effect is the intrinsic characteristic of coal heat generation and plays a significant role in the whole process of coal self-ignition. When a coal mass is exposed to air or another oxidising atmosphere at low temperatures, heat generation is primarily determined by the rates at which O 2 molecules can be physically and chemically adsorbed (this latter process equating to chemisorption) on the internal surfaces of micro-pores [13]. Chemisorption plays a more significant role in coal self-heating, due to larger heat generation than the physical adsorption at low ambient temperatures [5,14]. ...
... Conversely, low EOE-time values are associated with 'younger' coal that actively undergoes oxidation. Assuming that the O 2 consumption rate of coal per unit mass, Φ ON (mol/(kg·s)), is proportional to the heat generated, q hN [1,4,11,13]. Φ ON can be obtained from q N as ...
... is the reaction heat of coal oxidation [13]. ...
Article
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... In various practical scenarios such as open coal stockpiles, gob, mining-affected and outcrop coal seams etc., CSC is an intricate thermal dynamic phenomenon ranging from low-temperature coal oxidation to thermal runaway in case that the coal temperature exceed a threshold level [10,11]. The spontaneous combustion causes the alteration of composition and molecular structure in coals, accompanied with heat and mass transfer with release of gaseous products. ...
... The spontaneous combustion causes the alteration of composition and molecular structure in coals, accompanied with heat and mass transfer with release of gaseous products. Fig. 1(a) illustrates the heat and mass transfer process by Zhang et al. [11]. The coals prone to spontaneous combustion depend not only on coal reactivity and thermal stability, but very much determined by the physical factors including pore and particle feature, moisture phase change, surface freshness, availability of oxygen, ventilation condition and air humidity, and geological conditions etc. [12]. ...
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... According to coal-oxygen reaction theory, the energy inducing the CSC is predominantly from the interaction between coal and oxygen. The self-heating of coal occurs if the heat produced during oxidation exceeds the heat emitted to the environment [61,62]. Functional groups are the pivotal structural units of coal molecules. ...
... Schematic and major characteristics of CSC[61]. Process of CSC[62]. (Note: T s : Self-ignition temperature). ...
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Coal spontaneous combustion (CSC) is a major concern in the exploitation and utilisation processes of coal. Various technologies for inhibiting CSC have been developed, promoting industrial safety. With consideration of the complexity of the processes involved, efficient fire prevention and control techniques should be developed. Commonly used retardants for CSC prevention were reviewed. Based upon the necessary conditions required for CSC and coal − oxygen reaction mechanism, various materials used in CSC inhibition are analysed in detail and classified into physical-based, chemical-based, as well as composite retardants. The novel composite retardants combine both physical and chemical-based mechanisms in controlling CSC. The advantages and disadvantages of each CSC retardant are discussed and inter-compared. This review can promote research in the most needed areas to develop more efficient and affordable materials for CSC prevention.
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... In previous studies of coal-oxygen compound reactions, various parameters of coal sponta- neous combustion processes were studied using adiabatic oxidation tests, programmed heat experiments and gas chromatography [3][4][5][6][7][8][9][10][11]. Based on the coal-oxygen-compound theory, many scholars believe that the phenomenon of coal spontaneous combustion may generally be divided into three stages: the incubation stage, the self-heating stage and the combustion stage. ...
Article
Full-text available
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... Various strategies are employed to prevent self-heating, including compaction. Airflow within the pile and self-heating can be modelled, for example, using thermal computational fluid dynamics (CFD) simulations (Zhang et al., 2016). ...
Thesis
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... It not only causes wastage of resources (Shao et al. 2015;Wang et al. 2018a;Li et al. 2016), but also leads to gas and coal dust explosions, which threaten the health of mineworkers seriously. In addition, hazardous gas, as well as greenhouse gases, is generated (Yi et al. 2021;Xu et al. 2018;Honscha et al. 2021), thus, causing significant damage to the atmosphere and ecological environment (Wang et al. 2018b;Zhang et al. 2016;Song and Kuenzer 2014). Accurate prediction and forecasting are important to control the hazard of coal spontaneous combustion (Onifade and Genc 2020;Qu 2018). ...
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... The high levels of deep ground stress and ground temperature provide favorable conditions for coal self-heating, thereby increasing the risk of fire. The self-heating process is initiated by the increase of coal temperatures resulting from highly exothermic oxidation reactions during the exposure of coal mass to oxygen from mine air [3][4][5][6]. If the heat produced during the oxidation reaction accumulates, the temperature starts to increase up to the ignition point. ...
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Full-text available
Focused on the prediction and forecast index of coal spontaneous combustion, the temperature-programmed experiments were carried out to identify the releasing rule of the gaseous compounds with the consideration of coal particle sizes. Based on the method of growth rate analysis, critical and dry cracking temperature temperatures were determined, dividing the coal low-temperature oxidation process into three stages. The critical temperature for the spontaneous combustion of the coal is in the range of 60℃~70℃, with the dry cracking temperature ranging from 100℃ to 110℃. It was found that the particle sizes had obvious effects on the release of gaseous compounds during the coal heating test. To reduce the influence of coal particle size on the forecasting of coal spontaneous combustion, the combination methods of coefficient of variation and the weighted grey relational analysis were proposed for the selection and evaluation of coal spontaneous combustion indexes. For the tested coal sample, CO 2 /CO was selected as the first index, O 2 /(CO+CO 2 ) and CO 2 /O 2 as the second index. C 2 H 4 and C 2 H 6 were selected as the index gases to confirm that the coal oxidation reached the dry cracking temperature. This research could provide a reference for the determination of the grading forecasting index of coal spontaneous combustion.
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Oxidation of a bituminous coal has been studied using an isothermal flow reactor operated at temperatures between 60 and 90 degreesC, and packed with coal particles smaller than 853 mum in diameter. The time-dependent rates of production Of CO2 and CO during experiments were obtained simultaneously from the measurement Of CO2 and CO concentration at the reactor's exit, just after the onset of oxidation of the coal, using a dual-column micro gas chromatograph. The ratio of the rates of production Of CO2 and CO was high at the beginning of an experiment, but decreased to reach a steady state after 10 hours or so. Experimental data indicated that this ratio, at steady state, depends on temperature, and is independent of the size of coal particles and oxygen concentration in the oxidizing medium. The latter discovery has led to formulation of a new mechanism for low-temperature oxidation of coal, which is consistent with the existing spectroscopic data. It is proposed that carbon oxides are produced via the direct burnoff reaction and the decomposition of stable oxygenated complexes, such as carboxyl and carbonyl species. The decomposition of the initial chemisorption intermediates liberates carbon dioxide. For the First time, the paper reports an estimate of the activation energy for the direct burnoff reaction, Which ranges between 62.3 and 70.1 kJ/mol for the present coal. The proposed mechanism in Conjunction with the activation energy for the direct burnoff reaction could be used for predicting the emission of CO2 and CO from low-temperature oxidation of coal in practical applications.
Article
Laboratory experiments were conducted to investigate carbon monoxide (CO) and carbon dioxide (CO2) emissions from spontaneous heating of three U.S. coal samples in an isothermal oven at temperatures between 50 and 110 degrees C. The oxygen (O-2) concentration of an oxygen/nitrogen (N-2) mixture flowing through the coal sample was 3, 5, 10, 15, and 21%, respectively. The temperature at the center of the coal sample was continuously monitored, while the CO, CO2, and O-2 concentrations of the exit gas were continuously measured. The results indicate that the CO and CO2 concentrations and the CO/CO2 ratio increased when the initial temperature was increased. As the inlet O-2 concentration increased, the CO and CO2 concentrations increased, while the CO/CO2 ratios tended to converge to the same value. The ratio of CO/CO2 was found to be independent of coal properties, approaching a constant value of 0.2. The maximum CO production rate correlated well with the maximum coal temperature rise. The apparent order of reaction for coal oxidation was estimated to be between 0.52 and 0.72. The experimental results in this study could be used for early detection and evaluation of a spontaneous heating in underground coal mines. Published by Elsevier Ltd.
Article
A transient one-dimensional spontaneous heating model has been formulated to describe the spontaneous heating process at relatively low temperatures. The model consists of three differential equations describing the temperature, oxygen, and moisture concentration in a char pile. These equations have been solved numerically by a finite difference technique, and the influence of the initial char temperature, char reactivity, particle size, and moisture content on the process of spontaneous heating have been examined. Sensitivities are presented for the most important factors affecting the self-heating of coal, including a comparison of coal pile characteristics with and without moisture adsorption, desorption, and migration. This model was also validated against data from large-scale spontaneous heating tests.
Article
Understanding the evolution of coal self-heating in longwall mining gob areas is important for the mitigation of underground mine fires. The coal self-heating involves a chain of complex interactions between compositional gas and solid coal. These interactions are normally regulated by the dynamic gob configuration due to the extraction of coal. In this study, a fully coupled transient model of compositional gas flow and transport, and the heat transfer between solid coal and gas is developed to quantify the evolution of coal self-heating processes under the in-situ gob configuration. The modelled results were matched reasonably well with the field measurements of air temperatures and oxygen concentrations for a Chinese coal mine. The verified model was applied to conduct sensitivity studies of (1) ventilation flux; (2) ventilation resistance; and (3) advance rate of face. The main results of the sensitivity study reveal that (i) the high temperature zones are mainly distributed near the intake airway of gob; (ii) the higher ventilation flux or ventilation resistance leads to the higher self-heating temperature and the larger oxidation self-heating zone. Moreover, the self-heating zone migrates towards the much deeper gob area. Whereas, the higher advance rate of face results in the lower self-heating temperature and smaller self-heating zone. The simulated results can provide some suggestions for the prevention of coal spontaneous combustion in gobs.
Article
Pollutant emission characteristics and the synergistic interaction during low-temperature oxidation of blended coal were investigated. Two kinds of bituminous coals, high volatile bituminous coal (HC) and low volatile bituminous coal (LC), and their blends were heated from room temperature to 300 °C by 2 °C/min under air atmosphere, controlled and measured with the TGA–FTIR. The results showed that CO2 emission was affected by both moisture content and pore structure of coal. The blended ratio of 60HC40LC inhibited CO2 emission for the coupling effect of moisture content and pore structure. The minerals like sodium and potassium contributed to SO2 generation, while calcium and magnesium inhibited SO2 generation, thus the blended ratio of 40HC60LC inhibited SO2 emission with the synergistic interaction. What’s more, the blended coal could reduce HCN and HCl emission in different blended ratio. Thereby, blended coal technology was an effective way to reduce pollutant emission during low-temperature oxidation.
Article
In this study, combustion characteristics and kinetics of three coal samples are examined. A series of thermogravimetry (TG–DTG) and differential scanning calorimetry (DSC) measurements were carried out in non-isothermal conditions at heating rates of 5, 10, and 15 K min−1. Reaction regions, peak and burn-out temperatures, mass loss, and heat of reactions of the samples were determined for each heating rate from TG–DTG and DSC curves. A variety of computational methods, available in the software developed, were applied to experimental data for the evaluation of the kinetic parameters of the coal samples, and the results are discussed.
Article
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 degrees C. Moreover, the TGA-DSC method can also measure the kinetic parameters of coal oxidation at the temperatures from similar to 150 degrees C to that of ignition occurring, which were shown to be different from those at the temperatures below similar to 150 degrees 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.
Article
In order to investigate the effect of fine coal covering around the bottom of coal stockpile on spontaneous combustion prevention, a two-dimensional math model was established to numerically simulate the fine coal covering coal pile and a coal pile temperature-rising experimental system was setup to study the two-dimensional heat and mass transfer characteristics of air diffusion in the horizontal direction and air heat convection in the vertical direction inside coal stockpiles covered by different thicknesses of fine coal. The results showed that (1) the fine coal located at the bottom of the coal pile can effectively inhibit air convection and diffusion, cut off oxygen replenishment, and prevent the temperature rise inside coal pile, and (2) thicker fine coal has more obvious effect. Finally, the field experiments on the fine coal covering coal piles for preventing self-ignition of coal pile were carried out successfully. The results showed that the uncovered or exposed coal piles self-ignited rapidly within a very short period (18 days), while the coal pile covered with 1 m fine coal lasted for 123 days with a maximum coal temperature of only 59.9 °C. The characteristics of temperature distribution and diffusion inside coal stockpile were studied, and the high-temperature region was found in the region 1.3 m high and 2–3 m deep in the coal pile. The effects of environment temperature and precipitation on self-ignition of coal pile were also analyzed in field experiment. The experimental results proved that the fine coal covering technology can effectively prevent the spontaneous combustion of coal stockpiles.
Article
The shortcomings of some advanced models for heat and fluid flow and moisture transport within and around a reactive porous medium (a coal stockpile) are numerically investigated based on a local thermal non-equilibrium approach, and the results are compared with experimental data. A full account of the problem is presented, including transient variation of the maximum temperature in the medium and the time required to vaporize the water content. The former is the main parameter used in the study of spontaneous self-ignition of coal stockpiles, and the latter is an indication of the last step of self-heating toward self-ignition. Correlations are developed for the maximum dimensionless temperature inside the pile when the moisture content is near zero and the time that it takes from the beginning of self-heating until this time as functions of porosity, permeability, moisture content and the overall size of the porous medium.
Article
Results of an experimental and theoretical modelling investigation on the low-temperature oxidation of fresh, wet, low-rank coals are reported. The rates of oxygen consumption and carbon dioxide formation were measured for Wyoming subbituminous coal, using an isothermal fixed-bed flow reactor. The measurements are interpreted with a one-dimensional continuum model, including the single-particle diffusion-reaction model developed in Part 1, for the fixed-bed reactor. The model calculations are also compared with other data reported in the literature. The results indicate that the model provides a satisfactory explanation for the dependence on particle size and temperature of the low-temperature oxidation of dry and fresh wet coal. Extension of this formulation to model spontaneous combustion in coal stockpiles is reported in Part 3.
Article
Arrhenius parameters of oxidation of two Scottish bituminous coals were determined from heat-release rate measurements at laboratory oven temperatures. When the heat release rate expression so obtained was applied at stockpiling temperatures (∼300 K), one coal was shown to have a significantly greater propensity to spontaneous combustion than the other.
Article
This paper deals with the spontaneous combustibility and ignition behaviour of coal stored in confined spaces. The geometry, in which the stockpile is sealed on all sides except the top, which is exposed to the ambient conditions, is of interest in the transport of coals in barges or in rail cars. The role of oxygen diffusion as the cause of self-heating of coal is re-examined. In previous work, it was concluded that the diffusion in a one-dimensional stockpile can lead to a maximum temperature rise of ∼80 K; this conclusion is shown to be a consequence of the unrealistic boundary conditions imposed on the oxygen and energy balance equations. Simple calculations, with more realistic boundary conditions which reflect that the resistance to heat transfer may lie at the interface between the pile and the ambient, lead to a maximum possible temperature rise of ∼2150 K. Mathematical models are developed, assuming that the pile is isothermal, to determine the influence of various parameters—coal reactivity, stockpile dimensions and ambient conditions—on ignition behaviour. In particular, it is shown that storage of coal in this configuration is, at best, conditionally safe. Based on the calculations, methods which may be implemented to suppress spontaneous combustibility in confined storage are discussed. Application of the model is illustrated by detailed calculations pertinent to the transport of coal in barges or in rail cars.
Article
The underground coal seam fire in the Wuda, Inner Mongolia of china is one of the most serious coal fires in the world with a history over 50 years and endangers the neighboring downwind urban area. To investigate the potential mercury emission and migration from the coal seam fire, in situ real-time measurement of total gaseous mercury (TGM) concentration using Lumex RA-915 + mercury analyzer were implemented on the fire zone and the urban area. The results show an average TGM concentration of 464 ng m-3 in the fumes released from surface vents and cracks on the fire zone, which leads to an elevated TGM concentration of 257 ng m-3 (211-375 ng m-3) in the near-surface air at the fire zone and 89 ng m-3 (23-211 ng m-3) at the peripheral area. The average TGM concentration in the adjoining downwind urban area of Wuda is 33 ng m-3. This result suggests that the coal seam fire may not only contribute to the global mercury inventory but also be a novel source for mercury pollution in the urban areas. The scenario of urban areas being adjacent to coal seam fires is not limited to Wuda but relatively common in northern China and elsewhere. Whether there are other cities under influence of coal seam fires merits further investigation.
Article
Spontaneous combustion of coarse coal stockpiles in temporary coal storage yards was investigated numerically using COMSOL Multiphysics software. The main purposes of the numerical investigation were to identify the self-ignition characteristics of coarse coal stockpiles and formulate a theoretical model to predict the self-ignition time and locations of coarse coal piles. A mathematical model for self-ignition of coarse coal piles was developed and the process of spontaneous ignition of coarse coal stockpiles was simulated. The kinetic data of low-temperature oxidation reaction was obtained from the laboratory-scale experiments with bituminous coals taken from Jindi Coal Mine of Shanxi Province in China. The influence of moisture was ignored because the studied coal had low moisture content (mass concentration: 1.87%) and both coal and ambient environment were assumed to be saturated with moisture (or ambient environment was assumed to be dry). The effects of five variables (i.e. wind velocity, oxygen concentration, height, porosity, and side slope) on the spontaneous ignition in coarse coal piles were examined. Simultaneously, a theoretical prediction model was formulated in light of variable analyses and a great number of simulations.
Article
The current paper presents experimental investigations as well as numerical simulations on the influence of water and humidity on the self-ignition of combustible bulk materials.It is well known, that bulk materials may undergo self-ignition if stored under specific conditions. In some cases, large amounts of these materials are exposed to a humid surrounding, e.g. dried coal in a moist atmosphere. Due to the effects of condensation and adsorption of water, additional heat is generated and transported into the bulk material. If the pile is stored slightly below its self-ignition temperature, the bulk material can become supercritical and an ignition occurs.Experiments were carried out for German lignite coal sampled in two different particle size fractions. They showed, that subcritical deposits turned to supercritical behaviour if the relative humidity in the surrounding was suddenly increased or water was poured on the surface of the sample. Besides the experiments, a numerical model was established to describe the effects of self-heating until ignition of the deposit, including the transportation of moisture. Simulations with this model led to satisfying results when compared to the experiments.
Article
Spontaneous ignition of coal stockpiles and its suppression in a coal storage yard are studied numerically. The process of coal ignition is simulated with several physical and chemical properties. The main purposes of the numerical simulation are to pursue novel suppression methods and verify them. Without losing essential features of spontaneous ignition, we adopt as simple numerical models as possible. The numerical approach is validated by comparison with the experimental data. The ignition mechanism is analyzed and essential elements for ignition are pursued. Based on the ignition mechanism, three new methods are proposed in this study. They are to adopt internal walls installed inside the pile, air blowing from the bottom of the pile, and a dual barrier installed at the front and the rear of the pile. Each method has been verified to retard spontaneous ignition time more fundamentally than the existing methods and the expected additional delay is about 10 to 30 days. But, air blowing is effective in suppression of spontaneous ignition only at higher rates than a critical rate. More delay can be made by a combined application of these methods. It is found that the proposed methods are viable and effective in delay of coal ignition.
Article
Coal fractions of different size distributions exhibited different ratio, ash and sulphur contents, and surface structures. This was confirmed using two low-sulphur and two high-sulphur bituminous coals. The effect was much less pronounced for low-sulphur coals than for high-sulphur coals. A significant difference in properties was noted between the two high-sulphur coals in spite of similar basic compositional parameters. This was confirmed by the fractal dimensionality factor D of Illinois No. 6 coal, which exceeded the theoretical value.
Article
The chemical reactions which occur when Illinois No. 6 (hv C) and Rawhide (SBB C) coals are thermally pretreated at 100°C and when Illinois No. 6 coal is subsequently oxidized at 100°C with O2 have been studied using in-situ FT-i.r. differende spectroscopy. Significant spectroscopic changes were seen. Vacuum drying at 100°C resulted in the decomposition of carboxylic acid species to form a variety of new carbonyl species (in Rawhide) and decarboxylated or decarbonylated coal (Illinois No. 6). Oxidation of predried Illinois No. 6 coal leads to the formation of new carbonyl species. The assumption that drying does not alter the chemical composition of coal may not be correct. Thus, overall spectroscopic (and chemical) changes observed in moderate temperature reaction studies may depend upon sample pretreatment, drying and storage. In addition, the time/temperature profile used in a reaction study may affect the overall changes observed by altering the relative contribution of the different reactions.
Article
The low-temperature oxidation of a Yallourn brown coal was found to decrease markedly the internal surface area of the coal. The surface areas were determined by applying the Dubinin-Polanyi approach to carbon-dioxide isotherms measured gravimetrically at 0 °C.
Article
The high moisture content of Victorian brown coals is the major obstacle to their economic utilization. Some insight into the bonding of this water in the coals can be obtained from the water sorption isotherms reported in this investigation. The isotherms (30–60 °C) are typically sigmoid with a marked desorption/adsorption hysteresis extending to low relative vapour pressures. The BET monolayer capacity of the coal for water, calculated from the isotherms, correlates with the number of hydrophilic functional groups on the coal rather than the extent of its surface. Isosteric heats of sorption determined from the isotherms increase from the latent heat of vaporization (10.4 kcal/mol∗) in the bulk water or saturated end of the isotherm to 14 kcal/mol in the monolayer region. This increase can be explained by enhanced hydrogen bonding of the water to functional groups on the coal surface. A mechanism for the hysteresis effect has been proposed, based on differences between the desorption and adsorption data.
Article
A new set of dimensionless parameters and variables is defined with a view to simplifying and clarifying the theoretical determination of the initial ambient and the critical initial temperatures in spatially uniform self-heating systems. With these new variables the determination of the critical ambient temperatures can be achieved without the iterative procedure necessary when using the traditional variables with the Arrhenius function.The determination of the critical initial temperature using the new variables highlights a significant new source of error which arises when the exponential approximation arises. There is neither necessity nor advantage in making this approximation with these new variables.
Article
Buben's theoretical treatment of ignition and extinction in heterogeneous reactions is extended to reaction in porous solids. The theory is used to interpret experimental results on the self-ignition of wood sawdust in different concentrations of oxygen. The effect of the finite rate of diffusion of oxygen was estimated to have increased the Frank-Kamenestky critical ignition parameter by only nine per cent for the wood sawdust in an external concentration of oxygen as low as four per cent v/v. The rate of heat evolution prior to ignition increased with the two-thirds power of the external oxygen concentration at a given temperature. The reduction of ambient temperature required to extinguish combution in the sawdust, in four per cent oxygen, was less than predicted from the initial ignition characteristics of the sawdust. This effect is ascribed to a chemical properties of the sawdurst following ignition.
Article
An experimental kinetic study was made on the oxidation of bituminous coal and the results were used to test the mathematical model presented in Part I. Satisfactory agreement was observed between the data and the derived rate expressions, which included a proposed mechanism change above 230°C. The CO2/CO ratio at each temperature was found to be constant over a wide range of operating conditions.
Article
A general rate expression based on a proposed dual-path reaction model is developed for the oxidation of coal under mild conditions such as are useful in pretreatment for gasification. The effects on the oxidation rate of mass transfer, particle surface, oxygen partial pressure, and temperature have been considered and incorporated into the derived rate expression.
Article
In transient self-heating problems the spatial distribution of temperature in symmetrically heated bodies is usually assumed to be uniform and an effective surface transfer coefficient is defined so that the analysis can be simplified. This paper shows how this effective heat transfer coefficient can be simply related to the real surface cooling.
Article
A mathematical model is presented to describe steady-state mass transfer and oxidation processes in coal at low temperatures in a fixed-bed flow reactor. The model incorporates the effects of partial pressure of oxygen, temperature and coal particle size, and accounts for the rate of coal oxidation and the composition of oxygenated products at high pressures. This is an important development since previous models did not include the effect of pressure in their formulation. It is found that, when the partial pressure of oxygen increases the rate of oxygen consumption increases accordingly. However, the influence of partial pressure of oxygen on the rate of oxidation is less pronounced when the pressure surpasses 1MPa. In addition the model predicts that, for a constant partial pressure of oxygen, higher rates of oxygen consumption occur at lower total pressures. The same trends are also found for the concentration of oxygenated products at the reactor outlet. It is suggested that, the variation of partial pressure of oxygen leads to different concentration levels of oxygen at the surface and within the pores of coal particles, substantially affecting the rate of oxidation.
Article
A commercial CFD software programme, FLUENT, was used to study the oxidation process of coal in the mined-out longwall (gob) area. A three-dimensional, single-phasemodel with a continuously advancing longwall face has been developed. For the model, the gob longwall area was designed on the basis of the actual longwall panel operating in the Ostrava-Karviná Coal Mines (OKD, Czech Republic). The behaviour of the coal to oxygen was modelled using the results arising mainly from the former laboratory-scale experiments with Czech bituminous coals. Basically, the technique of pulse flow calorimetry and measurements at a continuous airflow reactor were applied during the laboratory investigations. In the contribution, the main focus was to understand the effect of the longwall face advancing speed on the oxidation heat production as well as evolution of the gases in the gob area. Simultaneously, the effect of coal crushing in the mined-out area on the spontaneous heating process was examined.Numerical simulations confirmed the existence of a “favourable” zone for the onset and development of the spontaneous heating process in the gob area. The location and the maximal temperature reached in the “favourable” zone were found to be significantly affected by the advancing rate of the coalface. The slower the advancing rate is, the higher the maximal temperature and smaller the depth of the “favourable” zone in the gob area are. When the rate drops to a certain “critical” value, spontaneous heating turns to flammable combustion of the coal. The value of the “critical” advancing rate was confirmed to increase if the grain size of the coal left in the gob decreases. Numerical examinations of carbon monoxide concentrations then proved that small incidents of spontaneous heating could occur in the gob area that need not be detected in the airflow of the longwall tail gate.
Article
Low-temperature oxidation of four different-rank Turkish coals was studied in order to assess the effects of temperature, particle size, coal petrography, and coal rank by monitoring CO2 and CO formation rates and calculated CO/CO2 ratios. Coals were classified to −425+212, −212+150, and −150+106μm size groups and oxidized at 40, 60, and 90°C. Temperature has a pronounced effect on the formation rates of CO2 and CO; when the temperature increases, the formation rates of CO2 and CO also increases. Nevertheless, the formation rates of CO2 and CO were found to be independent of particle size, particularly at relatively lower temperatures. Petrographic analysis showed that the coal sample having the highest inertinite group macerals was oxidized more easily, thus, yielding more CO2 and CO. Relatively higher rank coals were oxidized more easily, but oxidation diminishes with time. On the contrary, oxidation progresses with time for lower rank samples especially at relatively higher temperatures.
Article
An adiabatic calorimeter has been designed to enable the spontaneous combustion propensity of coal to be established. The experiment was designed to run unattended, with a personal computer being used for measurement and control functions. All measurements are stored on a data diskette while the experiment is in progress. The calorimeter was designed to be run in both a rising temperature mode and and an incubation mode. Various indicators of self-heating potential, such as total temperature rise, initial rate of heating, minimum self-heating temperature, and kinetic constants can be investigated. Results obtained from the adiabatic tests will be compared with the results of crossing-point temperature determinations and differential thermal analysis (DTA) tests for the same coals, with a view to formulating a mathematically consistent spontaneous combustion liability index. This paper describes the major components of the adiabatic calorimeter.
Article
Previously related publications by the authors have identified the spontaneous heating in underground coal mines as a combination of the seam factor, the geological factor and the mining factor. In this publication the authors pay particular attention to the effects of the seam factor from research conducted from the end of the nineteenth century to the deductions of modern day workers.
Article
An overview is given of the possibilities thermal analysis and in particular thermogravimetry has in studying low-temperature oxidation of coal. First, procedures are described with which representative samples of coal can be obtained. Secondly, the oxygen consumption of coal has been measured using a sensitive vacuum microbalance, coupled to a mass spectrometer to analyze the effluent gas. Finally, with infrared spectroscopy structural changes at the coal surface, due to low-temperature oxidation are determined.
Article
Critical conditions are obtained for ignition in a self-heating solid system consisting of two components generating heat independently, one component being inexhaustible and the other exhaustible by either simple first order or autocatalytic reaction. Ignition depends on whether the exhaustible component can cause a temperature rise in excess of the upper stationary, but unstable, value possible for the inexhaustible component reacting alone. The system provides a theoretical model for some commonly occuring examples of self-heating and ignition in porous solids containing oxidizable oils.
Article
The problem of autoignition by the absorption of moisture has been postulated to occur in a number of situations of high practical importance. In this article we examine this phenomenon from a theoretical point of view. The problem is formulated as a combination of classical criticality theory and the critical initial value problem as defined more recently. The authors show that the phenomenon of wetting-induced ignition (WII) is possible in certain regions of parameter space, but however close the dry material may have been to criticality before wetting a finite amount of water is required to cause ignition. Equally interesting is the finding, that for a given material, WII is impossible in sufficiently small samples but possible in larger ones. This reveals an important flaw in scaling test procedures.
Article
It is shown that experimentally-measured temperature profiles in a porous medium with heat generation along a line source can be modelled in terms of conduction and convection. The model predicts that convection always occurs for non-zero heat generation and that the concept of a critical Rayleigh number is not appropriate in this case. Using the model it is possible to calculate the volume of air flowing into the bed. It is concluded that natural convection is a feasible mechanism for oxygen transport into a coal dump where sufficient reaction may occur to sustain a hot spot.
Article
It has been known for a long time that combustible bulk materials may undergo selfignition if stored in sufficient amounts at adequate ambient temperatures. Most of the studies according to this problem refer to atmospheric ambient conditions (oxygen volume fraction of 21%). In some technical applications, however, bulk materials are processed or stored at a reduced level of oxygen concentration. Besides the oxygen volume fraction, the moisture content of the bulk material itself as well as the humidity of the ambient air effect self-ignition. The first part of the current paper presents experimental investigations on the influence of the volume fraction of oxygen on the self-ignition temperature of the bulk materials. The lower the oxygen volume fraction in the surrounding gas, the higher is the self-ignition temperature as a general trend. In the second part, tests under a normal atmosphere were carried out to investigate the influence of water in a liquid and vapour state on the self-ignition procedure of lignite coal. They showed that sub-critical deposits could become super-critical by pouring water into the bulk or by exposing a dry bulk sample to humid air. Besides the experiments, a numerical model was established which allows to compute the process of heating and self-ignition in bulk deposits. Technical applications of the model cover safe storage of dusts, granulate, bulk materials and wastes.
Article
Spontaneous combustion can occur in a coal stockpile if the heat generated by oxidation cannot be dissipated at near ambient temperature. Determination of conditions for which combustion occurs is of great importance in designing coal stockpiles. An approximate analysis is used to obtain natural convection patterns in a laterally-unbounded layer of coal. Down-hexagons and two-dimensional rolls appear to be the stable flow planforms. A continuation procedure gives a simple criterion for the point of ignition in the layer in terms of easily measurable parameters. This criterion can be used to determine ignition points in the interior of a large coal stockpile and complements earlier work in which a similar criterion was developed for the edge of a stockpile.
Article
In the paper, we present a mathematical model including a detailed radiation balance of the coal stockpile surface. Our aim was to describe the radiation balance in more detail even on the oriented slopes of a stockpile. Here, the periodic boundary condition differs from that applied on the horizontal surface. The energy from the Sun has a strong influence on the temperature profile in a coal stockpile. It would not be suitable to describe it by a constant, because it represents day and year cycles. The model is flexible enough to be applied to any part of the world since it uses meteorological input data obtained experimentally.Four types of coal were studied with different reactivity. We observed the influence of several factors (coal reactivity, coal matrix porosity, meteorological conditions, coal reactivity (two-dimensional model), coal matrix porosity (two-dimensional model)) on the temperature profile in two types of coal stockpiles. One-dimensional model can be applied on a situation when coal is being stored under ground. In this case only the topside is exposed to the ambient conditions. A two-dimensional model is necessary when describing a commonly used heap-like stockpile. Here, the changing boundary conditions must be applied on the topside as well as on both slopes. Finally, a long-term simulation for average meteorological conditions at a certain place was studied.
Article
Five coals of different rank and of different content of sulphur were subjected to oxidation by peroxyacetic acid (PAA), 5% nitric acid, by oxygen in 0.5N Na2CO3 aqueous solution and the air oxidation for 7 days at 125°C. The processes of oxidation were carried out for coal samples demineralised by the Radmacher method, and additionally for the pyrite-free coal samples. Proximate, elemental and spectral analyses of the initial coals and the products obtained from them were made. The most effective oxidising agents were 5% HNO3 and PAA. As a result of oxidation, a significant part of the organic components of coal is converted into acid soluble products. Depending on the oxidising agent, the loss of sulphur in the solid oxidation products was different and the highest for the coal samples oxidised with HNO3 and PAA. Formation of oxidised sulphur species (SO and –SO2) was detected by IR spectroscopy. FTIR data also provide useful information on evolution of the molecular structure of different rank coals on oxidation, in particular in the carbonyl and aliphatic ranges (1800–1500 and 3500–2800cm−1).