The electrochemical method can strengthen gas desorption and seepage from coal. The study on change of the pore-fracture structure of coal after electrochemical modification can help to reveal the mechanism. Anthracite was modified by the electrochemical method using our own self-developed experiment apparatus. The pore-fracture structure of modified samples was measured by micro-CT. Combined with the Matlab software, its characteristics such as pore number, porosity, and average pore diameter were analyzed. The results show that (1) the number of fractures in modified coal samples increases. The shape of new fractures in samples in the anodic and cathodic zones was irregular voids and striola, respectively. The effect of electrochemical treatment on the section of samples close to the electrode is relatively obvious. (2) With increasing pore size, the number of pores in samples changes according to negative exponential rules. After electrochemical modification, the porosity of modified samples in the anodic zone increases from 11.88% to 31.65%, and the porosity of modified samples in the cathodic zone increases from 12.13% to 36.71%. (3) The main reason for the increase in the number of pores of coal samples in the anodic and cathodic zones is the treatment of electrolytic dissolution of minerals and electrophoretic migration of charged particles, respectively.
The coal-bed methane has attracted widespread attention as clean energy. However, permeability of coal seams is very low—less than 0.001 mD [
The methods of improving permeability mainly include protective seam mining [
In the aspect of the pore-fracture structure test of coal, the micro-CT technology is widely used because of nondestructive measurement and direct observation of the inner structure [
Anthracite was modified by the electrochemical method using our own self-developed experiment apparatus. The pore-fracture structure of modified samples was measured by micro-CT scanning. Combined with the Matlab software, its characteristics such as pore number, porosity, and average pore diameter were analyzed, and the change mechanism of coal structure was revealed.
Anthracite was collected from the No. 15 coal seam in the southeast of Qinshui Basin from Sihe coal mine (Shanxi province, China). Two cylindrical specimens with a diameter of 24.83 mm and a length of 25 mm and two cylindrical specimens with a diameter of 2.54 mm and a length of 10 mm were processed using the drilling machine, cutting machine, and polisher machine. The samples were scanned using a micro-CT device and then placed near the anode and cathode in the electrochemical modification apparatus, respectively. The electrolyte was Na2SO4 solution with a concentration of 0.05 mol·L−1. The electric potential gradient was 4 V·cm−1. The action time was 120 h. The modified samples were cleaned with distilled water and dried in an oven at 378–383 K until a constant weight was achieved. The samples were scanned at the same test condition using the micro-CT device. In addition, maximum vitrinite reflectance measurements and proximate, ultimate, and maceral composition and coal ash composition analyses were performed following the GB/T 6948-2008, GB/T 212-2008, GB/T 476-2001, GB/T 8899-1998 and GB/T 1574-2007 standard procedures, respectively. The test results are shown in Tables
The proximate, ultimate, and maceral composition of anthracite.
Coal samples | Romax (%) | Proximate (%) | Ultimate (%) | Maceral (%) | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Mad | Aad | Vad | C | H | O | S | V | I | L | ||
The original | 2.86 | 1.65 | 5.21 | 6.12 | 86.52 | 2.64 | 6.83 | 3.32 | 86.3 | 13.7 | 0.0 |
Modified sample near anode | 2.73 | 0.68 | 3.15 | 8.21 | 85.20 | 2.72 | 8.77 | 2.01 | 85.7 | 14.3 | 0.0 |
Modified sample near cathode | 2.89 | 0.76 | 4.08 | 8.67 | 85.68 | 3.14 | 7.32 | 2.76 | 84.1 | 15.9 | 0.0 |
Note: Romax = vitrinite reflectance; V = vitrinite; I = inertinite; L = liptinite.
The test result of coal ash composition.
Coal sample | Coal ash composition (%) | ||||||||
---|---|---|---|---|---|---|---|---|---|
SiO2 | Al2O3 | CaO | Fe2O3 | SO3 | MgO | TiO2 | Na2O | P2O5 | |
The original | 31.86 | 23.77 | 19.01 | 15.93 | 7.46 | 0.66 | 0.94 | 0.32 | 0.05 |
Modified sample near anode | 37.83 | 28.31 | 12.56 | 12.32 | 5.29 | 0.37 | 2.31 | 0.63 | 0.38 |
Modified sample near cathode | 26.25 | 18.39 | 22.93 | 19.43 | 8.72 | 1.83 | 1.29 | 0.74 | 0.42 |
It can be seen that the ash content of samples modified in the anodic and cathodic zones decreases from 5.21% to 3.15% and 4.08%, respectively. The major reduction in chemical compositions of the modified samples in the anodic zone is CaO (from 19.01% to 12.56%), Fe2O3 (from 15.93% to 12.32%), and SO3 (from 7.46% to 5.29%). The major reduction in chemical compositions of modified samples in the cathodic zone is SiO2 (from 31.86% to 26.25%) and Al2O3 (from 23.77% to 18.39%).
The electrochemical modification equipment shown schematically in Figure
Diagram of the electrochemical modification apparatus.
The micro-CT system mCT225kVFCB was developed jointly by Taiyuan University of Technology and Chinese Academy of Engineering Physics (Figure
Miro-CT measurement system.
The scanned CT images are gray images. The gray levels can reflect the density of the scanned material. The brighter a certain part of the image is, the higher the density of material in this zone is. The white, black, and gray areas represent minerals, pores, and coal matrix, respectively. Thus, the pore structure of coal can be obtained. In order to reflect the change characteristics of the pore-fracture structure of coal after modification, the sample was divided into 3 sections on the direction vertical to the bedding plane (Figure
Schematic diagram of trisection along the direction vertical to the bedding plane of coal.
Figure
CT image of the structure of coal in the anodic area after modification. (a) Top section, (b) middle section, and (c) bottom section.
After electrochemical modification, the inner structure of coal samples in the anodic zone shows three kinds of change: (1)some new fractures and cavities appear, especially around filled mineral, such as zones A1 and A2 in Figure
Figure
CT image of the structure of coal in the cathodic area after modification. (a) Top section, (b) middle section, and (c) bottom section.
The statistical result of the size of coal matrix.
Number | Length (mm) | Width (mm) |
---|---|---|
A1 | 10.06 | 4.07 |
A2 | 7.71 | 5.82 |
A3 | 7.74 | 4.15 |
Average | 8.50 | 4.68 |
The maximum statistical area of the scanned sample with a diameter of 2.54 mm was segmented from the single
CT image of the inner structure of anthracite with a diameter of 2.54 mm before and after modification. (a) Unmodified anodic zone, (b) modified anodic zone, (c) unmodified cathodic zone, and (d) modified cathodic zone.
In order to obtain the change rule of the pore structure of the samples modified in anodic and cathodic, respectively, the porosity is calculated and the numbers of connected groups are counted. The segmented image is being processed by binaryzation, colorInvert, connected group label, and regional colorization, as shown in Figure
The distribution of the connected group of anthracite with a diameter of 2.54 mm after modification. (a) Unmodified anodic zone, (b) modified anodic zone, (c) unmodified cathodic zone, and (d) modified cathodic zone.
Pore structure parameters of anthracite with a diameter of 2.54 mm.
Name | Porosity (%) | Total pore number | Maximum pore size ( |
Average pore size ( |
---|---|---|---|---|
Unmodified anodic zone | 11.88 | 6045 | 50.08 | 6.09 |
Modified anodic zone | 31.65 | 3314 | 117.94 | 20.43 |
Unmodified cathodic zone | 12.13 | 6521 | 52.33 | 7.71 |
Modified cathodic zone | 36.71 | 2896 | 159.77 | 32.77 |
In order to obtain the pore size distribution, the Count the number of pixels in labeled connected groups. Calculate the area of connected groups according to the area of pixels, and then obtain the pore diameter by equivalent conversion. Count the number of connected groups which have the same counts of pixels, and then obtain the number of pores which have the same pore size and the total pore number, as shown in Table Calculate the equivalent average diameter of pores according to the total area of pixels and the number of pores, as shown in Table
Pore size distribution of samples with a diameter of 2.54 mm before and after modification. (a) Sample modified in the anodic zone; (b) sample modified in the cathodic zone.
Anthracite is a semiconductor [
In addition, as can be seen in Tables
Schematic diagram of the mechanism by which anthracite was modified by the electrochemical method.
The decreased ash composition of the samples modified near the cathode is mainly SiO2 and Al2O3. It may be resulted from electrophoretic migration of clay. The particles (coal powder and clay) which filled generally in pores and coal matrix show negative charge in alkaline solution. Under the action of the electric field, these charged particles move to the anode, which increase the number of pores and provide passage for gas migration. Guo [
The number of fractures in modified coal samples increases. The shape of new fractures in samples in the anodic and cathodic zones was irregular voids and striola, respectively. The effect of electrochemical treatment on the section of samples close to the electrode is relatively obvious. With increasing pore size, the number of pores in coal samples changes according to negative exponential rules. After electrochemical modification, the porosity and average pore diameter of coal matrix increase. The porosity of modified samples in the anodic zone increases from 11.88% to 31.65%, and the porosity of modified samples in the cathodic zone increases from 12.13% to 36.71%. The main reason for the increase in the number of pores of coal samples in the anodic and cathodic zones is the treatment of electrolytic dissolution of minerals and electrophoretic migration of charged particles, respectively.
The authors declare that there are no conflicts of interest.
This research was supported financially by the National Natural Science Foundation of China (Grant no. 51174141) and the Basic Research Project of Shanxi Province (Grant no. 201701D221241).