Dielectric characterization of coals

Forniés-Marquina, J.M.; Martín, José Carlos; Martínez, Javier Puertas; López‐Miranda, José; Romero, C.

doi:10.1139/p02-125

Cited by 24 publications

(13 citation statements)

References 9 publications

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“…Therefore, after the raw coal loses water, the coal dielectric constant reaches a minimum. Carbon-carbon π-bonds exist between aromatic carbon layers, and the electron of the π-bond in crystalline carbon was a type of localized electron [39][40][41] . Localized electrons remain in localized sites most of the time, but can occasionally jump to neighboring localized sites, which could lead to polarization under certain boundary conditions.…”

Section: Changes In the Functional Groups Of Oxidized Coal The Type mentioning

confidence: 99%

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

Zhu

Wang

et al. 2020

Sci Rep

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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 oxygencontaining 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 T 2 to T 3. Coal spontaneous combustion is one of the major disasters in collieries, and it had occurred in countries all over the world. Researchers reported that many of the coal spontaneous combustion areas were concealed 1,2. Accurate detection of the high-temperature zone in the early low-temperature oxidation stage is a major interest within the coal spontaneous combustion prevention and control field. Electromagnetic detecting technology is a conventional coal fire detection technology. Some researchers had made progress in locating the coal fire area using magnetotelluric field monitoring equipment. However, this method is greatly affected by the content of magnetic mineral components in the coal seam, especially pyrite 3. Some other researchers 4,5 had tried to locate coal fire by ground-penetrating radar technique based on voids and clefts induced by coal fires. Magnetic mineral components do not restrict this method, but it may be affected by formation fractures. The limitations on electromagnetic detection of coal fire may be reduced if we can find the mapping relationship between coal dielectric constant and temperature in low-temperature oxidation stage. The prediction time may be advanced as well. It is believed that the dielectric properties of coal are the basis for studying the propagation of electromagnetic waves in the coal seam. The dielectric properties are also the critical parameters for the development of various geological exploration instruments (e.g., ground-penetrating radar, transient electromagnetic (TEM) instruments, etc.) 6-10. Therefore, it is of great theoretical and practical value to study the coal dielectric properties under low-temperature oxidation condition for the detection of concealed coal low-temperature oxidation areas. Many researchers had reported the di...

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Section: Changes In the Functional Groups Of Oxidized Coal The Type mentioning

confidence: 99%

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

Zhu

Wang

et al. 2020

Sci Rep

View full text Add to dashboard Cite

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“…(Balanis et al 1980;Marland et al 2001;Forniés-Marquina et al 2003). Thus, it is hard to accurately measure the dielectric properties and the test results obtained by different scholars are usually inconsistent.…”

Section: Test Of Complex Dielectric Constant Of Coalmentioning

confidence: 99%

“…Forniés-Marquina et al (2003) has shown that when the sulphur content was higher than 4 % the coal relaxation frequency would decreased with the increase of sulphur content, which might contribute to microwave absorption.…”

Section: Responses Of Coal and Sulphur-containing Constituents To Micmentioning

confidence: 99%

Progress of the technique of coal microwave desulfurization

Tao

Xie

et al. 2014

Int J Coal Sci Technol

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With the advantages of its fast speed, effective and moderate controllable conditions, desulfurization of coal by microwave has become research focus in the field of clean coal technology. Coal is a homogeneous mixture which consists of various components with different dielectric properties, so their abilities to absorb microwaves are different, and the sulfur-containing components are better absorbers of microwave, which makes them can be selectively heated and reacted under microwave irradiation. There still remain controversies on the principle of microwave desulfurization at present, thermal effects or non-thermal effects. The point of thermal effects of microwave is mainly base on its characters of rapidly and selectly heating. While, in view of non-thermal effect, direct interactions between the microwave electromagnetic field and sulfur containing components are proposed. It is a fundamental problem to determine the dielectric properties of coal and the sulfur-containing components to reveal the interaction of microwave and sulfur-containing compounds. However, the test of dielectric property of coal is affected by many factors, which makes it difficult to measure dielectric properties accurately. In order to achieve better desulfurization effect, the researchers employ methods of adding chemical additives such as acid, alkali, oxidant, reductant, or changing the reaction atmosphere, or combining with other methods such as magnetic separation, ultrasonic and microorganism. Researchers in this field have also put forward several processes, and have obtained a number of patents. Obscurity of microwave desulfurization mechanism, uncertainties in qualitative and quantitative analysis of sulfur-containing functional groups in coal, and the lack of special microwave equipment have limited further development of microwave desulfurization technology.

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“…It has also been determined that high-pressure water vapor (microwave frequency 2.45 GHz, microwave power 1 kW, irradiation time 240 s, coal sample temperature 222 °C, relative atmospheric pressure 1.54 kPa) in the pore structure may cause pore deformation, destruction, and interconnection [20,21]. The moisture present in the coal reservoir displays a dielectric loss greater than that in the coal matrix; therefore, it may be selectively heated using microwaves [22][23][24]. We have also previously shown that the water contained in coal rocks can be quickly heated under microwave irradiation.…”

Section: Introductionmentioning

confidence: 99%

Enhancement of Coal Permeability Using Aqueous NaCl with Microwave Irradiation

Guan

Nan

et al. 2022

Geofluids

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Water in coal pores can generate strong steam explosions when treated with microwave irradiation. In order to improve the sensitivity of coal reservoirs to microwaves, we used varying concentrations of NaCl on the pore solution, which further increased the effect of microwaves on permeability enhancement. In our experiments, we selected 3 ratios of water content and 5 different concentrations of NaCl. The changes of coal water content and permeability before and after irradiation were compared. In addition, changes in coal surface temperature and internal thermal power were analyzed through thermal imaging and random sphere numerical modeling. Results showed that the increase of the proportion of solution mass and the ion concentration in the solution improved the overall dielectric properties of coal rock samples. After microwave irradiation, the average reduction rate of water content in coal rock samples increased with the increase of ion concentration in the solution. Both the average surface temperature of the coal rock samples and the average electromagnetic heating power gradually increased; this increases the temperature rise rate and maximum temperature of coal rock samples under the same irradiation time, which is conducive to better rapid accumulation of steam pressure, thereby improving the pore structure more effectively. Finally, the maximum growth rate of permeability reached 466.18%. This work provides a novel train of thought to enhance coal permeability by using microwave irradiation.

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Dielectric characterization of coals

Cited by 24 publications

References 9 publications

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

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

Progress of the technique of coal microwave desulfurization

Enhancement of Coal Permeability Using Aqueous NaCl with Microwave Irradiation

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