3.1 Tests and analysis of breakdown voltage and breakdown field intensity
Breakdown field intensity is a physical variable that reflects electric strength of dielectric substances. There are three breakdown forms for solid dielectrics[25-27]:
(1) Electric breakdown is the phenomenon that electric field makes dielectric lose the insulting properties by accumulating enough quantity and energy of charged particles in it.
(2) Thermal breakdown means that dielectric loses insulating properties upon excessive temperature caused by accumulation of heats in an electric field.
(3) Electrochemical breakdown means that dielectric develops chemical changes slowly as a response to the collaborative effects of electric field and temperature, which result in gradual deterioration of performances and finally losing insulation properties.
During breakdown voltage test of coal samples, there’s no heat accumulation due to the relatively low preset-voltage, continuous and adjustable growth rate of voltage, and short experiment period. Therefore, electric breakdown of coal samples took the dominant role in the experiment. Due to the tight fitting between red copper electrode plates and coal samples in the holder, the space between two electrodes was 100.00mm. Three breakdown experiments were performed to coal samples of high, moderate and low metamorphic grades, respectively. The electric field intensity between electrodes can be calculated as Equation (1):
where E is the electric field intensity between electrodes (kV/cm), U is voltage between electrodes (kV), and d is the space between two electrodes (cm).
Test results of breakdown voltage and breakdown field intensity of different coal samples are listed in Tab.1.
Table.1 Test results of breakdown voltages and field I ntensities of different coal samples
Coal type
|
Sample No.
|
Cadf
%
|
Break downvoltage
kV
|
Breakdown field intensity
kV/cm
|
Resistivity
Ω·m
|
Lignite
|
HM-1
|
57.021
|
9.3
|
0.93
|
5763.15
|
HM-2
|
58.436
|
8.5
|
0.85
|
5689.49
|
HM-3
|
59.087
|
7.9
|
0.79
|
5417.23
|
Long-flame coal
|
CYM-1
|
74.035
|
7.5
|
0.75
|
3928.35
|
CYM-2
|
75.213
|
6.4
|
0.64
|
3727.14
|
CYM-3
|
77.852
|
7.6
|
0.76
|
4192.12
|
Anthracite coal
|
WYM-1
|
87.265
|
5.0
|
0.50
|
3435.17
|
WYM-2
|
90.714
|
5.7
|
0.57
|
3624.22
|
WYM-3
|
89.532
|
4.2
|
0.42
|
3237.54
|
it can be seen from table 1 that the higher the resistivity between the same coal samples and the smaller the fixed carbon content, the higher the breakdown voltage. The resistivity of different coal samples decreases with the increase of metamorphic degree. In addition under the same experimental conditions, breakdown voltage and field intensity differ significantly among different coal samples even though the coal size is identical. Breakdown field intensity is determined by breakdown voltage. The distribution pattern of breakdown voltage was drawn to reflect differences of breakdown voltage among different coal samples clearly and intuitively (Figure.2).
According to tests and analysis, coal samples at different metamorphic grades show significantly different breakdown voltage and breakdown field intensity. This is because coals are inhomogeneous media with dual pore characteristics and there’s uneven distribution of mineral in coals[28]. The primary cracks and mineral distribution in solid media can affect breakdown voltage significantly [29]. Therefore, breakdown voltage of the same coal type with same size might vary to some extent. The average breakdown voltage and breakdown field intensity of lowly metamorphic coals are higher compared to those of highly metamorphic coals. Reasons can be explained as follows. With the increasing degree of coalification, carbon content in coals increases and electrical conductivity of coals might soar up continuously when the carbon content increases to 92%[30], finally resulting in the breakdown of highly metamorphic coals under a relatively low voltage.
At the same time, the coal conductivity is not only related to the degree of coal metamorphism, but also closely related to the internal temperature of the coal[31]. Relevant scholars have studied the law of coal body resistivity changing with temperature, and found that when the coal body temperature is between 30℃ and 90℃, the resistivity decreases linearly with the increase of temperature. The voltages 2kV and 4kV applied in this paper are both below the breakdown voltage of the coal body. The purpose is to test the electrical parameters and surface temperature rise based on the coal body structure without damage.
In this work, we used infrared thermal imager to test the surface temperature change of coal. Under such condition, we assumed that the local temperature of any carbon-carbon connection between the particles in the sample will not lead to any chemical reaction (and additional carbonization) that changes the internal carbon structure.
3.2 Variation laws of currents in coals under the action of an applied DC field
Coal is a typical dielectric material. According to types of electrical conductivity, coals can be divided into electron conduction type, ionic conduction type, electron-ionic mixed conduction type and hole conduction:
(1) Electron conduction is based on a certain amount of free radicals in molecular structure of coals. These free radicals gain energies from the applied electric field to get rid of original binding and jump along the electric field, forming electron flows.
(2) Ionic conduction is mainly attributed to the high contents of water and mineral in coals. These mineral contain ionic compounds which can form positively charged ions that can move freely under the action of water, thus forming currents along the electric field.
(3) Mixed conduction means that dielectric conduction contains both ionic conduction and electron conduction. These three conduction types often coexist in coals, with differences in the dominant conduction type.
(4) Hole conduction means that some valence electrons of covalent bonds in coal gain some energy due to thermal movement, and thus get rid of the constraints of covalent bonds and become free electrons. At the same time, holes are left on the covalent bonds, and the holes have net surplus. The positive charge will attract other electrons around to fill the holes, and then form holes to move in the crystal to conduct electricity. [32]
Coal samples in this experiment were processed by vacuum drying and degassing before current test to eliminate water and gas. As a result, electron conduction took the dominant role in coal samples. According to test and analysis, changes of currents in different coal samples with time in the applied DC field were disclosed, which demonstrated and verified variation laws of electrical parameters of coal samples (Figure.3).
Coal is a unique dielectric material and free electrons in coals change from scattering to directed arrangement under the action of applied DC field, forming moving current beams. With the increase of voltage, these electrons are easier to get rid of binding from groups and thereby form more electrons in the free excited state, thus increasing the current continuously. Therefore, it can be seen from the I-t curve in Figure.3 that given the same loading voltage, currents in all coal samples increase significantly with the increase of the conduction time. Specifically, when the loading voltage is 2kV, currents in lignite increases to the maximum 2,259μA from the initial 787μA. Currents in long-flame coal increase from the initial 864μA to the maximum 2,827μA. Moreover, currents in anthracite coal increases from the initial 913μA to maximum 3,212μA. When the loading voltage is 4kV, currents in lignite increases to the maximum 3,845μA from the initial 1,483μA. Currents in long-flame coal increase from the initial 2,060μA to the maximum 5,013μA, and currents in anthracite coal increases from the initial 2,422μA to 6,704μA.
The changes of currents which run through coal samples are attributed to changes of resistances, which were caused by the changes in the local microcrystalline connections. Therefore, dynamic variation characteristics of currents in coal samples can reflect variation laws of electrical parameters of coal samples in applied electric fields indirectly. In this experiment, the growth amplitude and rate of currents in anthracite coal are higher than those of long-flame coal and lignite. This is because there’s small binding strength to electrons in molecular structure of anthracite coal and the possibility of electron transition is increased significantly due to the relatively high ordered aromatic degree[33].
As the loading test continued, currents in all coal samples reached the “inflection points”. This is related with heat exchange with the external world. When joule heats generated by currents in coal samples are equivalent to the escaping heats to the outside, conduction characteristics of coal samples and currents that run through coal samples became stable. Accordingly, specific resistance of coal samples became basically stable.
In Figure.3(a), lignite, long-flame coal and anthracite coal reach the inflection points at 55min, 50min and 45min when the loading voltage is 2kV. In Figure.3(b), lignite, long-flame coal and anthracite coal reach the inflection points earlier at 45min, 40min and 35min when the loading voltage is 4kV, respectively. These reveal that increasing loading voltage of coal samples can make coal samples reach the inflection point of peak current earlier.
3.3 Temperature rise effect on coal surface in an applied DC field
Under the action of an applied DC field, alkane side chains and functional groups with low bond energy on coal surfaces break and fall off, which causes macromolecular structural changes and generate new micromolecular gases on coal surfaces [34,35]. Coals can generate pyrolysis gases like CH4, H2 and CO when they are heated to 200℃ in inert atmosphere[36-38]. Therefore, studying temperature rise effect on coal surface under the action of an applied DC field is the key to distinguish gas production from “electrochemical” effect from gas production from pyrolysis caused by joule heating effect.
Coal is a typical inhomogeneous multiphase medium. When excessive voltages are applied at two ends of coal samples, a plasma channel is formed in coal samples to break coals under the collaborative influences of electric field stress and thermal expansion stress[17]. In this experiment, the loading voltages at two ends of coal samples were maintained below breakdown voltage, which were inadequate to breakdown of coal samples. However, internal and surface temperatures of coal samples will surely increase under the continuous action of an applied DC field due to the joule heating effect. According to the Lenz’s law[12] and specific heat formula[39], the relationship between temperature rise and loading voltage of coal samples can be disclosed. The power loss of coal samples under the action of an applied DC field is:
Where U is the applied voltage (kV), R is the equivalent resistance of coal samples (Ω), c is specific heat capacity of coal samples (J/kg·°C), m is mass (kg, and k is the coefficient of heats transformed from power loss and it is dimensionless.
According to test and analysis in Section 3.2, anthracite coal shows the highest growth rate of current and amplitude of peak current under the same loading voltage. Therefore, anthracite was chosen to analyze temperature rise effect on coal surface in an applied DC field in the following text. During the test, temperatures on coal surface were tested every 5min since a DC field was applied onto the coal sample for 10min. On this basis, the variation curve of highest temperature on anthracite coal surface with time was drawn (Figure.4).
In a 4kV applied DC field, temperature on anthracite coal surface experiences three stages (Figure.4), which are the slowly warming stage (10~25min, section AB), rapid warming stage (25~45min, section BC) and slow cooling stage (45~90min, section CD).
Temperature rise on coal sample surface is a dynamic process. Moreover, it is found from the experiment that temperature distribution on coal sample surface is uneven. The infrared distribution cloud map of temperatures on coal samples in above three stages were collected (Figure.5-Figure.7) to characterize and reflect the continuous variation of surface temperature accurately.
(1) Slowly warming stage (Section AB)
It can be seen from Figure.4 that the highest temperature on anthracite coal surface increases slowly within the section AB (10~25min) in an applied DC field. In this stage, the highest temperature changes between 30.2℃~34.4℃, the temperature variation is △T=4.2℃, and the average temperature rise rate is Tv=0.28℃/min. Distribution patterns of temperature on coal surface and variation laws in the slowly warming stage are shown in Figure.5. Obviously, there’s uneven distribution of temperatures on coal surface, manifested by local heat accumulations. Meanwhile, it can be concluded from a comparative analysis of Figure.5(a)~(b) that the surface temperature on anthracite coal increases slowly. This is related with the small current and weak joule heating effect in the beginning of loading test when coal samples are controlled by resistance effect.
(2) Rapid warming stage (Section BC)
It can be seen from Figure.4 that slope of the variation curve of highest temperature on anthracite coal surface increases significantly within the section BC (25~45min) in an applied DC field. In this stage, the highest temperature changes between 38.4℃~65.3℃, the temperature variation is △T=26.9℃, and the average temperature rise rate is Tv=1.79℃/min. Similarly, there’s uneven distribution of temperatures on coal surface. Due to accumulation of loading time and joule heats in the experiment, electrical parameters of coal samples are changed and equivalent resistance declines. As a result, currents that run through coal samples increase greatly, thus resulting in the rapid temperature rise on anthracite coal surface.
(3) Slow cooling stage (Section CD)
It can be seen from Figure.4 that in the section CD (45~90min), the highest temperature on anthracite coal surface reaches the peak at 45min, then decreases slowly within 55~75min, and finally becomes stable after 75min in an applied DC field. In this stage, the highest temperature changes between 65.5℃~63.5℃, the temperature variation is △T=-2.0℃, and the average temperature rise rate is Tv≈-0.13℃/min. According to the infrared temperature distribution cloud map on coal sample surface, temperature is also distributed unevenly (Figure.7). In this stage, temperature on coal sample surface declines slowly and then becomes stable, finally fluctuating at about 63℃. At this moment, the joule heats generated by currents that run through coal samples are equivalent to the escaping heats from the experimental system to the external environment, so that temperature on coal sample surface and electrical parameters tend to be stable.
According to a comparative analysis of Figure.5~7, temperature changes on anthracite coal surface in an applied DC field have following characteristics.
(1) There’s uneven temperature distribution on coal sample surface. In the temperature rise process, regions with high temperature is close to the upper ends of coals. This is related with the uneven mineral distribution in coal samples and different specific heat capacity of different minerals.
(2) According to a comparative study on I-t curves of anthracite coal samples in a 4kV applied DC field (Figure.3(b)), variations of the highest temperature and temperature rise effect on coal sample surface lag behind changes of currents that run through coal samples for 5~10min.
(3) In the process of temperature rise on anthracite coal surface, the highest temperature is lower than 70℃, which is far beyond the lowest temperature (200℃) for pyrolysis-based gas production of coals. Therefore, gas production of coals in an applied DC field is not attributed to pyrolysis.