Process Of Coal Spontaneous Combustion Research Articles

A study on the prediction method of coal spontaneous combustion development period based on critical temperature.

Prediction of the development and evolution process of coal spontaneous combustion is of great significance to its prevention and control. Through the testing of different metamorphic degrees coal programmed heating characteristics and the infrared functional groups, the critical temperatures of the coalification process were analyzed and a method to predict the period of spontaneous combustion was established according to the characteristic of the critical temperature change. What can be inferred from this research is that the critical temperature of activation energy of coal in programmed heating experiments can characterize the nature of coal at low temperature and the macrocritical temperature point can be obtained from the characteristics of the activation energy change during the low-temperature period. The critical point of coal functional groups in terms of its micro changes is similar to its macro changes, the absorption intensity of the characteristic peak shows a general increasing trend, with an initial decline and then increase, followed by a decrease and a subsequent increase. In the high-temperature combustion process, the activation energy is not constant and changes from a negative value to 0, then the activation energy changes to a positive value, and finally goes down to a negative value. Therefore, the prediction of development period of spontaneous combustion can be achieved through the critical temperature of activation energy which acts as the critical temperature in high-temperature combustion process, and the prediction of low-temperature oxidation process is based on the critical temperature of activation energy in programmed heating process and the macro critical temperature of absorption intensity of coal functional groups.

Influence of granularity on thermal behaviour in the process of lignite spontaneous combustion

Granularity is one of the primary extrinsic factors that can influence the process of coal spontaneous combustion. Lignite was selected for studying the influence of granularity. A self-developed temperature-programmed oil bath experimental system was adopted to capture the changes in the spontaneous combustion characteristic parameters for different particle sizes. Moreover, the microcrystalline structure and functional group distribution of different granularities were obtained using an X-ray diffractometer and Fourier transform infrared spectrometer. The results show that the microcrystalline structure, the types of functional groups and the release order of gases of different granularities have equal influence during the process of coal spontaneous combustion. A smaller granularity has a lower aromatic group content and a higher content of high-activity hydroxyl groups, aliphatic hydrocarbons and oxygen-containing functional groups. Moreover, a smaller particle size results in a lower critical temperature. The distributions of coal granularity also affect the characteristic of coal spontaneous combustion. A wide particle size distribution produces a larger surface area for contact between coal and oxygen than a narrow distribution, which results in a coal with a stronger spontaneous combustion tendency.

Study on test method of heat release intensity and thermophysical parameters of loose coal

Accurate determination of basic parameters of coal spontaneous combustion process can not only directly or indirectly characterize the spontaneous combustion of coal, but also provide a reliable theoretical basis for the accurate prediction of coal spontaneous combustion period and the proposition of prevention measures of spontaneous combustion. In this paper, based on the theory of porous medium heat transfer, the mathematical models of parameters such as oxidative heat release intensity, thermal conductivity, heat capacity and activation energy of loose coal were established by using element balance method. Then, an integrated experiment system for testing characteristic parameters of coal spontaneous combustion was designed and set up, and the characteristic parameters of loose coal spontaneous combustion were accurately tested through the spontaneous combustion heating experiment on large-volume coal. The results show that the thermal conductivities of loose coal samples all decrease with the increase of voidage and rise with the increase of metamorphic grade. The heat capacity grows with the increase of volatile content. The oxidative heat release intensities of samples were increased exponentially with the rising temperature, which first rises slowly in the temperature range of 60–70 °C and then increases rapidly after 70 °C. The sample with a lower metamorphic grade and a stronger spontaneous combustion tendency owns greater oxidative heat release intensity, which grows with the increase of dynamic absorbed oxygen. At the same temperature, the large-size sample has smaller oxidative heat release intensity. The activation energy of coal samples increases with the rise of temperature and metamorphic grade. The experimental results indicate that the test system and method are reliable and can achieve the goal of rapid test.

Study on the change of organic sulfur forms in coal during low-temperature oxidation process

Organic sulfur functional groups in coal not only have certain oxidation characteristics at low temperature and but also can break to produce free radicals, which is bound to have a considerable effect on coal spontaneous combustion. However, this area has not been reported yet. There are relatively few studies on variation laws of organic sulfur functional groups in coal in the coal spontaneous combustion process (especially low-temperature oxidation stage), and the release of SO2, the gaseous product of organic sulfur during the low-temperature oxidation, is also rarely reported. Therefore, it is necessary to systematically obtain the change laws of organic sulfur forms in coal during low-temperature oxidation. In this paper, the morphological changes of organic sulfur in coal during low-temperature oxidation were studied first, and then the changes of organic sulfur forms in coal before and after low-temperature oxidation and the release of the low-temperature oxidation product SO2 were studied by using the X-ray photon spectroscopy (XPS) technique and the XLZ-1090 infrared gas analyzer. The XPS analysis results show that the relative contents of mercaptan and sulfide are reduced; the content of thiophene sulfur decreases slightly or remains unchanged; and the contents of sulfone, sulfonate and sulfate all increase. This verifies that the low-temperature oxidation process of active organic sulfur functional groups in coal is: sulfide → sulfoxide → sulfone, mercaptan → disulphide → sulfoxide → sulfone → sulfonic acid. The study reveals that the release of gaseous SO2 is not detected during the low-temperature oxidation of the raw coal samples, whereas it is detected during the low-temperature oxidation of the acid-treated coal samples. This is because the acid treatment can remove alkaline minerals in coal, weakens the inhibitory effect of alkaline minerals on SO2 and enable SO2 to escape almost completely. In addition, the study also discovers that the time and amount of SO2 gas release during the low-temperature oxidation process are directly related to the metamorphic degree and organic sulfur content of coal.

Experimental investigation on the macroscopic characteristic parameters of coal spontaneous combustion under adiabatic oxidation conditions with a mini combustion furnace

ABSTRACTResearch on macroscopic characteristic parameters of coal spontaneous combustion under adiabatic oxidation conditions helps in the early detection of self-ignition of coal oxidation even coal fires. To achieve this aim, a mini combustion furnace for studying self-ignition of coal was designed and constructed. The coal sample came from Huang Lin coal mine, Shaanxi province, China. Then, the variations in temperature, gas ratios, oxygen consumption rate, heat intensity, and limit parameters were studied. Results show that during the process of coal oxidation, coal temperature and gas concentrations inside the furnace vary with the change of time, air leakage intensity, and heat dissipation condition. The variation trends of macroscopic characteristic parameters during the entire process of coal spontaneous combustion in a certain range of temperature under various oxygen concentrations can be applied for temperature prediction and fire prevention of coal.

Characteristics of carbon monoxide production and oxidation kinetics during the decaying process of coal spontaneous combustion

AbstractLaboratorial experiments of coal spontaneous combustion under non‐isothermal conditions were conducted using a self‐made instrument aimed at investigating the characteristics of carbon monoxide emissions and kinetic characteristics. The impacts of coal rank, particle size, and oxygen content on the CO production during the decaying process of spontaneous combustion were studied. The experimental results demonstrated that coal particle size had a significant influence on the spontaneous combustion at high temperatures and coals of a large particle size decreased slowly in terms of the total CO production. Meanwhile, it is also found that even if the oxygen content was as low as 3 %, the oxidation reaction of coal still existed, liberating CO and heat at the same time. Therefore, stricter standards were recommended when judging whether or not a spontaneous combustion had been extinguished in gobs. Additionally, the studies on oxidation kinetics of coal implied that the activation energy was dependent both on the oxidation temperatures and coal properties. Oxidation temperatures affect the magnitude of activation energy, while coal properties impact the response of activation energy to the oxygen contents.

Experimental study on the influence of different oxygen concentrations on coal spontaneous combustion characteristic parameters

Oxygen concentration of the oxidation combustion zone in coalfield fire zone and coal spontaneous combustion dangerous area in coal mine such as goaf and sealed fire zone are all lower than that of normal atmospheric. Also, these two areas have the characteristics of high temperature. Therefore, in this paper, XKGW-1 type high temperature coal spontaneous combustion experiment device had been designed and manufactured independently. It was employed to investigate the entire process of coal spontaneous combustion by using the test method of temperature programmed. Five oxygen concentrations, 21, 17, 9, 5, and 3 vol%, were tested, observed, and elucidated. The changing tendency of coal spontaneous combustion index from room temperature to 450.0°C high temperature was explored under those oxygen concentrations. Experimental results showed that the effect of the oxygen concentration on the characteristic index of coal spontaneous combustion was very intricate and complex. The impacting result and degree varied with the range of the temperature. Consequently, during the practical application combined the effecting characteristics of the variable oxygen concentrations on coal spontaneous combustion into the on-site condition will significantly benefit the plan of the fire prevention. [Received: January 18, 2016; Accepted: April 19, 2016]

Thermodynamic characteristics of coal reaction under low oxygen concentration conditions

In order to further understand the characteristics of coal reaction under low oxygen concentration atmosphere, this study tested the thermodynamic characteristics during reaction processes under low oxygen concentrations. The kinetic factors were also analyzed based on testing results. The results show that there are some characteristic temperatures during coal spontaneous combustion and the influences of oxygen concentration on these temperatures are different. The influences of oxygen concentration on the mechanism function and kinetic parameters depend on the coal ranks and the reaction stages. They are different at different reaction stages for the same coal, and at the same reaction stage for different coals. This phenomenon is essentially resulted from the different influences of oxygen concentration on the micro reaction sequences during coal spontaneous combustion. The study will be helpful for further understanding of the actual development process of coal spontaneous combustion.

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.

Numerical Simulation and Backfilling Materials Research on Coal Spontaneous Combustion of Thick Seam Large-Scale Top-Caving Region in Resources Conformity Coal Mine

This paper aimed at fire control in thick seam large-scale top-carving region. On the basis of coal and oxygen compounding theory, theoretical analysis, numerical simulation and experiment are combined, and a coal spontaneous combustion process model is built according to fluid mechanics and control theory. By studying and testing on top-carving coal spontaneous combustion process, conclusion is drawn that spontaneous combustion area is the largest in partly-closed region, followed by unclosed region. A totally closed baffle leads to the smallest spontaneous combustion area and the smallest possibility of fire. With local materials in a certain condition, new, cheap backfilling materials are developed. Thus provide theoretical basis for study on the forecasting and prevention of thick seam large-scale top-carving coal spontaneous combustion.

Evaluation of the propensity for coal spontaneous combustion based on catastrophe theory

Generally, different prevention measures should be taken according to spontaneous combustion propensities. The current methods to evaluate the propensity of coal spontaneous combustion, such as chromatographic method of oxygen adsorption, oxidation kinetics method and activation energy method, are mostly affected by human factors. Their boundaries among different classes of propensities were all established by subjective judgments. A new evaluation method using catastrophe theory is introduced. This method can accurately depict the process of coal spontaneous combustion and the evaluation index, “catastrophe temperature”, be obtained based on the model. In terms of catastrophe temperature, the spontaneous combustion propensity of different coals can be sequenced. Experimental data indicate that this method is appropriate to describe the spontaneous combustion process and to evaluate the propensity of coal spontaneous combustion.

Research on the Coupling Effects Between Stereo Gas Extraction and Coal Spontaneous Combustion

Abstract In order to investigate the effects of the stereo gas extraction and overlying strata fracture development on the coal spontaneous combustion, through the research of air leakage channel and coal oxidative dynamic, we applied the U+II type fully machined workface of Shigang mine to study on the coupling between the stereo gas extraction and overlying strata fracture development, which caused air leakage and spontaneous combustion. The results show that the stereo gas extraction and overlying strata fracture development support air leakage passage and dynamic oxidation to coal spontaneous combustion in goaf and aggravated the process of coal spontaneous combustion. The study provides a new notion of controlling coal spontaneous combustion in the high gas and easily spontaneous combustion coal-seam of fully machined workface, which is of a certain theoretical and practical significance.

Study on Coal Spontaneous Combustion Characteristic Structures Affected by Ionic Liquids

According to modern coal chemical structure theory, the active chemical structure categories in coal spontaneous combustion process were summed up, which show that the coal spontaneous combustion characteristic structures are dynamic, such as methyl, methylene, hydroxyl, carbonyl and others in coal bridge bonds and side chains. The amount and activity of these structures play an important role in coal low temperature oxidation stage. Based on this, the article analyzed the effect of ionic liquids to coal spontaneous combustion characteristic structures, which indicates that the structure amount of coal affected by ionic liquids decreases obviously compared with raw coal.

Study on Small Simulation Device of Coal Spontaneous Combustion Process

Considering advantages and disadvantages of present ignition stations, the double nested spontaneous furnace with better thermal insulation performance was developed, and electrical sand bath was taken as thermal insulation and gas preheating device, also the temperature acquisition and temperature control device with higher accuracy was selected in order to solve the problems of heat dissipation and temperature control. The ignition furnace and sand-bath attemperator of the device form the double insurance to protect the heat in the process of the coal spontaneous combustion reaction, and could guarantee the coal spontaneous combustion reaction process to be simulated scientifically and veritably. With this device, the coal sample of the SHIGANG COAL MINE was experimented to simulate the spontaneous combustion reaction process, proving that the device is feasible. The device can be used in the relevant experiments of coal spontaneous combustion.

Analysis of mine’s air leakage based on pressure gradient matrix between nodes

Air leakage may significantly affect the effectiveness of mine ventilation by increasing the cost of ventilation and arousing problems for ventilation management. Furthermore, air leakage may accelerate the process of coal spontaneous combustion and cause gas explosion, thus greatly threatens the safety of coal production. The estimation of air leakage, therefore, have great practical significance. For any ventilation system of coal mines, there is a defined pattern of pressure gradient which drived the mine air to flow in the network, drives possible air leakage to go shortcut as well. Air leakage may occur through ventilation structures such as ventilation doors and fractures of the surroundig coal and rock of airways. A concept and the relevent calculation method of the pressure gradient matrix was put forward to assist the analysis of potential air leakage routes. A simplified example was used to introduce the application principle of pressure gradient matrix in identifying all the potential air leaking routes, which offers a deeper understanding over the ventilation system and the prevention of coal spontaneous combustion.