Investigation of Second Contraction Difference between the Active Components and the Inertinite

Abstract

The method for separating the active component and the inertinite, respectively, in a lump coal was investigated, and it was confirmed that they could be separated from each other from microscopic observation. Then the methods for measuring the contraction of the active component and the inertinite were investigated. The author made a rod-shaped sample from a lump semicoke of the active component and the inertinite heated to 773 K for the measurement of contraction. Using rod-shaped samples had good results. The contraction of the active component was higher than that of the inert, confirming the trend that the higher the degree of coal carbonization, the lower the contraction of the active component.

1. Introduction

It is said that supply and demand for coking coal will be tight in the future, and it is necessary to expand resources such as using low-quality coal. However, Blending these coals will reduce coke strength. Coke is a porous material, and the coke strength determined by the strength of matrix and the pore structure.^1,2) Microlithotype of coal is roughly classified into the active component and the inactive component (inertinite hereafter). In the carbonization process, it is known that the active component and the inertinite have different contraction rates, and when the difference in contraction rate is large, cracking occurs in coke, which causes a decrease in strength.³⁾ Therefore, it seems to be important for the understanding of coke strength expression to grasp each contraction rate difference and clarify the generation mechanism of crack around the inertinite.

As a separation method of both, coal was pulverized to a constant particle size, and then concentrated by gravity separation using the inertinite is heavy. However, in this method, there were many problems such as that complete separation is difficult, such as concentration of ash on inertinite side, and that coal is oxidized in a process of gravity separation and water washing, which may affect a carbonization process, and that a sample can be collected only in a powdery. Especially when measuring the contraction ratio in the powder sample, it is difficult to accurately measure such as thermal expansion of the particle gap. Therefore, the relationship between the particle size^4,5) and the inertinite size⁶⁾ obtained by microscopic observation concentrated coke and the coke strength (DI) was discussed.

The relationship between enrichment ratio and contraction ratio from 973 K to 1273 K of inertinite-concentrated coke had also been reported.⁷⁾ However, when measuring contraction in powder samples, it was considered difficult to accurately measure the thermal expansion of the inter-particle voids.

The relationship between enrichment ratio and shrinkage of inert-enriched charcoal has also been reported.⁷⁾ However, when measuring shrinkage in powder samples, it is considered difficult to accurately measure the thermal expansion of the inter-particle voids.

Then we devised the method to measure the contraction ratio of both. As a separation method of both, it was examined whether they could be separated by discriminating between them by using the difference between the appearance and hardness of the active component and the inertinite and the properties in the carbonization (expansivity of the active component and inertness of the iniertinite), After separation, we devised a method to measure contraction after separating the two and removing the effect of inter particle voids. This paper reported the result, and discussed the cause of the difference in contraction rate.

2. Experiment

2.1. Samples

Table 1 shows the coal properties used for measuring.

Table 1. Coal properties.

Coal	ASH	VM	logMF	Ro	ST	TI	C	H	N	O
	%	%	logddpm	%	°C	vol%	d%
Coal A	10.3	20.0	1.91	1.28	498	25.7	79.6	4.4	1.5	4.5
Coal B	6.8	35.9	4.41	0.88	494	14.8	79.1	5.2	1.7	6.2
Coal C	9.0	36.3	2.38	0.71	476	14.1	75.5	5.2	2.0	7.8

2.2. Taking Sample of Active Component and Inertinite

In order to prevent over-grinding, lump of coal was coarsely pulverized with a hammer or the like. Since the active component generally has a soft and black shiny, soft part that are split by hand were taken, and the inertinite took hard parts without shiny.

2.3. Sample Preparation for Measuring Contraction

The soft parts and hard parts were carbonized in lumps on an alumina boat. Semicoke was prepared by heating them to 773 K, which is higher than the re-solidification temperature at 3 K/min using a tubing furnace.

2.4. Observation Microscope

To make a coke core, coke was impregnated with epoxy resin and vacuum degassed. Coke cores were polished and observed using polarizing microscopy (LEICA: DM2500P).

2.5. Contraction Measurement

Contraction measured by thermal analyzer manufactured by NETZSCH Japan: TMA4000SA. The semicoke was formed into a cylinder with a diameter of 6 mm for use in TMA measurements. After replacing the gas from air to nitrogen gas in the system for 1 hour at 200 ml/min sample was heated from room temperature to 773 K is 20 K/min, from 773 K up to 1473 K was heated at 3 K/min, while contraction measured under the conditions of the load 50 g using piston with a diameter of 6 mm.

2.6. Measurement of Pyrolysis Gas

Pyrolysis gas measured by TG-MS manufactured by Rigaku; Thermo Mass. Approximately 10 mg of the 75 μm pulverized sample was heated to 1273 K under a He 300 ml/min flow at a heating rate 40 K/min, and the weight loss and the ionic strength of the pyrolysis gas was measured scanned ranging from m/z=2 to 45.

2.7. NMR Determination

AVANCE III 400 made of Bruker was used for measuring solid-state ¹H-NMR, and at room temperature, the resonance frequency was measured at 100.5 MHz with a relaxation time 5.0 s and a pulse width corresponding to a 90 pulse of 3.4 μs. The rotational speed of the sample was 1.3 kHz, and cumulative number was set as 256 times. The sample was ground to below 0.25 mm so that it could be filled into the zirconia rotor. The obtained spectra were separated using 4.7 ppm (H₂O) as aliphatic hydrogen on the high field side and aromatic carbon with hydrogen on the low field side.

2.7.1. ¹³C-NMR Determination

The device described above was used for measuring solid-state ¹³C-NMR, and the resonance frequency was measured by 100.5 MHz, DDMAS method. The measuring temperature was set at room temperature, and the rotational speed of the sample was set at 6000 times of 1.3 kHz, cumulative number. Samples were ground to below 0.25 mm so that the zirconia rotor could be filled as in ¹H-NMR. The resulting spectra were corrugated using the analysis programmed (wsola _1.3). The assignment of ¹³C-NMR spectra referred to this paper.⁸⁾ Aromatic carbon with hydrogen (Ar-H) and Bridgehead (bridgehead carbon numbers in polycyclic aromatics) in polycyclic aromatics were derived by the Eqs. (1) and (2) because of their very close spectral positions and difficult to separate waveforms. The aromatic condensation degree index χb (hereinafter, mean aromatic size) was calculated from the analytical values obtained by elemental analysis and ¹H-NMR, ¹³C-NMR (Eq. (3)).⁴⁾   

$$\mathrm{A}\mathrm{r}\text{-}\mathrm{H}=\mathrm{A}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{H}\times \left(\mathrm{H}/1\right)/\left(\mathrm{C}/12\right)$$(1)

  

$$\mathrm{B}\mathrm{r}\mathrm{i}\mathrm{d}\mathrm{g}\mathrm{e}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{d}=(\mathrm{B}\mathrm{r}\mathrm{i}\mathrm{d}\mathrm{g}\mathrm{e}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{d}+\mathrm{A}\mathrm{r}\text{-}\mathrm{H})-\mathrm{A}\mathrm{r}\text{-}\mathrm{H}$$(2)

  

$$\chi \mathrm{b}=\mathrm{B}\mathrm{r}\mathrm{i}\mathrm{d}\mathrm{g}\mathrm{e}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{d}/\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n}\mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}$$(3)

3. Results and Discussion

3.1. Discriminating Optical Texture of Coke by Polarizing Microscope

Polarizing microscope images of the the inertinite part of Coal A are shown on Fig. 1. Coal A showed a slight incorporation of a few micrometers of vitrinite organization (yellow) which exhibited around the porosity, but almost all of them were the inertinite texture (magenta), and other coals also showed a similar tendency.

Fig. 1.

Polarization micrograph of coal A. (Online version in color.)

3.2. Investigation of the Method for Measuring the Contraction of Active Component and Inertinite

The authors investigated the measuring methods of contraction of the active component and the inertinite using Coal C. As a sample for measuring contraction, three samples were prepared as follows: (1) a powder sample obtained by pulverizing a semicoke to 75 μm or less, (2) another sample was charged into a container having a diameter 10 mm, and a molded sample pressurized at about 180 kg/cm² from the upper portion, (3) a rod-shaped sample obtained by boring the sample from a semicoke lump to φ8 mm by a drill press, and then polishing the surface and horizontally leveling the sample. The semicoke mass of the active component had many voids and was fragile, and a rod-shaped sample could not be collected. The contraction rate determinations are shown on Fig. 2. Regarding the inertinite, it was confirmed that the contraction rate of the powder sample was as small as about half of that of the rod-shaped sample, and that the contraction rate changed discontinuously. This is since the voids between the particles generated by contraction of the particles remain at a defined load. In the sample subjected to pressure molding, contraction was started after once expanded to around 673 K. The reason is that the particles are compacted with each other, it is presumed that the air contained between the samples was thermally expanded without escaping. These phenomena were similar for the active component, and the difference in the contraction rate between the active component and the inertinite was also not clear. On the other hand, for the rod-shaped sample cut out from the inertinite lumps, the contraction rate is continuously changed, it is considered to be possible to accurately measure. Therefore, a rod-shaped sample was also prepared for the active component. Semicoke of the active component heated at 773 K and the active component pulverized to 75 μm or less was blended 3:7 ratio and heated to 773 K to prepare a semicoke mass in which growth of bubbles was suppressed, from which a rod-shaped sample was collected to measure the contraction ratio (Fig. 3). As a result, discontinuous contraction as seen in the powder sample was not generated even in the rod-shaped sample of the active component, and a clear difference could be confirmed in the contraction rate of the inertinite and active component, and the present test method was adopted as a contraction rate measuring method.

Fig. 2.

The contraction measurement result of various samples. (Online version in color.)

Fig. 3.

The contraction measurement result of various type of the active component. (Online version in color.)

3.3. On the Contraction Difference between Active Component and Inertinite

Figure 4 shows the contraction rate of the active component and the inertinite of various coal. The contraction rate of the inertinite was around 10% regardless of the brand, while the active component had a higher contraction rate compared to inertinite. And there was the difference between the coal, and Coal B showed the highest contraction rate.

Fig. 4.

The contraction measurement result of rod-shaped semicoke of the active component and the inertinite. (Online version in color.)

3.4. Regarding the Second Contraction of Coke

It is said that the second contraction of coke is highly related to the condensation behavior of aromatic rings, and hydrogen is generated in conjunction with the condensation of aromatic rings. Therefore, the hydrogen evolution behavior of the semicoke was calcined to 773 K of Coal A was measured by TG-MS and the results are shown in Fig. 5. DTG has a large peak derived from pyrolysis around 783 K, thereafter, the weight decreased slowly with the generation of hydrogen after about 823 K. Hence, 823 K was defined as the second contraction onset temperature of coke.

Fig. 5.

The measurement result of (a) thermo gravity and Mass Spectrum, (b) dTG. (Online version in color.)

3.5. On the Cause of the Contraction Rate Difference between Active Component and Inertinite

The inertinite showed no difference in contraction rate among the coal type, but significant difference in active component. Then the relation between the mean average reflectance and the second contraction of the active component and the inertinite of each coal is shown in Fig. 6. Coal with higher average reflectance tends to have lower second contraction for the active component. The degree of polymerization of the coal may have affected how the condensation progressed during the coking process. Therefore, the average aromatic size (χb) was evaluated using NMR to confirm the relation with the second contraction rate, respectively. The findings are presented in Fig. 7. The tendency of the second contraction rate to decrease for the coal with large χb could be confirmed, and this is thought to be due to condensation is in progress in the coal, resulting in fewer subsequent condensation, and smaller second contraction. In addition, as the gases generated after the re-solidification, most of them are hydrogen as described above, and it is known that they are generated mainly when aromatic carbon with hydrogen (Ar-H) is condensed, and it is considered that the second contraction ratio of the active component becomes higher as the amount of aromatic carbon with hydrogen (Ar-H) is larger. Therefore, the relationship between amount of aromatic carbon with hydrogen (Ar-H) and the second contraction ratio was confirmed (Fig. 8). In the same way, the integrated intensity of hydrogen (m/z=2) was measured by TG-MS, and the relation with the second contraction ratio was confirmed (Fig. 9). Coal with larger the amount of aromatic carbon with hydrogen (Ar-H) and the integrated intensity of hydrogen (m/z=2) showed a trend toward greater second contraction.

Fig. 6.

Relationship between average reflectance and the second contraction of the active component and the inertinite. (Online version in color.)

Fig. 7.

Relationship between aromatic cluster and the second contraction of the active component and the inertinite. (Online version in color.)

Fig. 8.

Relationship between aromatic carbon with hydrogen and the second contraction of of the active component and the inertinite. (Online version in color.)

Fig. 9.

Relationship between integral intensity of m/z=2 and the second contraction of the active component and the inertinite. (Online version in color.)

On the other hand, for inertinite, the χb and aromatic carbon with hydrogen (Ar-H) varied among charcoal species, but the contraction rate remained constant. It is known that the inertinite has a carbon structure like charcoal and does not show thermal plasticity but carbonizes in a solid state. Therefore, it is considered that the rearrangement and the development of the laminated structure accompanied by softening and melting as seen in the active component did not proceed, and the second contraction ratio was small because it carbonized and condensed while retaining an irregular carbon structure close to the difficult graphitization property, and there was no difference in the second contraction ratio between coals.

4. Conclusions

The method for separating the active component and the inertinite, respectively, in a lump coal was investigated, and it was confirmed that they could be separated from each other from microscopic observation.

The method for measuring the second contraction rate in the separated samples was examined, and good measurement results were obtained in the rod-shaped semicoke samples heated at 773 K.

As a result of measuring the second contraction rate of the activity component and the inertinite of various coal by the above method, the second contraction rate of the inertinite was almost constant regardless of the coal brand, but the active component was different for each coal.

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