Preparation of High-Strength Coke by Carbonization of Hot-Briquetted

ARTICLE pubs.acs.org/EF

Preparation of High-Strength Coke by Carbonization of Hot-Briquetted Victorian Brown Coal Aska Mori,† Sousuke Kubo,† Shinji Kudo,† Koyo Norinaga,† Tetsuya Kanai,‡ Hideyuki Aoki,‡ and Jun-ichiro Hayashi*,† † ‡

Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-0850, Japan School of Engineering, Tohoku University, Sendai 980-8579, Japan ABSTRACT: Cokes with tensile strengths of 6
37 MPa were prepared by binderless briquetting and subsequent carbonization up to 900 °C of pulverized Victorian brown coal that had little fluidity and very low reflectance. Application of mechanical pressures of 64
128 MPa at temperatures of 130
200 °C caused softening of the coal because of mobilization of both low- and high-molecularmass components, deformation of the coal matrix, and then coalescence/bonding of particles. The resulting coke had a density as high as 1.1
1.3 g/cm3 and tensile strength of 28
37 MPa, which was 5
6 times that of conventional metallurgical coke.

1. INTRODUCTION It is recognized that brown coal and lignite are not suitable feedstock of metallurgical coke that is generally produced from caking coal or its blend with slightly caking or noncaking coals. Victorian brown coals have reflectance and Gieseler fluidity of 0.19
0.36%1 and nearly zero, respectively, and these are both out of the ranges of the Miyazu
Okuyama
Fukuyama (MOF) diagram2 that gives composition of the coal blend suitable for producing high-strength metallurgical coke. Blending brown coal with caking coal may not be reasonable, if its particular nature, i. e., high absorptivity of fluid material causing the loss of fluidity of the blend upon heating,3,4 is taken into consideration. Formcoke technologies5
8 may allow for application of char/coke from brown coals to the production of coke briquettes if appropriate binders, such as raw or modified coal tars or asphalts, are available. This technology requires the pyrolysis/carbonization for producing not only char/coke but also a binder (in application of coal tar) before briquetting and carbonization. There have been reports on binderless briquetting of lowrank coals9
14 and successful production of coke from such briquettes.11
14 Bayraktar and Lawson13 briquetted an aciddemineralized Turkish lignite by applying 113
212 MPa mechanical pressure at ambient temperature and carbonized the briquette at 900 °C, thereby producing coke with a tensile strength of about 6 MPa. This strength was as high as those of conventional blast furnace and foundry cokes, 2
6 MPa.15
19 Rapid heating and in situ briquetting of coal at 350
400 °C, where coal can undergo not only physically induced but also pyrolysis-induced softening, successfully improved the mechanical strength of resultant coke and enabled the blending of poorcoking coal in the feedstock at fractions of as much as 50 wt %.11 Continuous application of mechanical pressure to non-caking coal while heated to 350
500 °C greatly improved strength of the resulting coke.12 It is believed that Victorian brown coal is the most suitable feedstock of binderless briquettes20 by virtue of its great affinity with water (moisture), which may play the role of plasticizer, lubricant, and/or adhesive, and, in addition, very low ash content. r 2011 American Chemical Society

The present authors propose binderless briquetting of brown coal at temperatures above 100 °C for further improvement of the mechanical property of briquette and resultant coke from the carbonization of the briquette. First, this method may enable the avoidance of the formation of defects that are associated with the moisture release in the early stage of heating toward carbonization without mechanical pressure. Simultaneous moisture removal and mechanical pressure application are expected to be effective in the maintenance of the adhesion/bonding of particles during subsequent heating. Second, applying mechanical pressure at temperatures of 100
200 °C may cause softening of brown coal over molecular to particle scales because of the thermal breakage of hydrogen bonds21 and mechanically induced mobilization of low-mass components. These events are expected to cause significant deformation and adhesion of the particles by playing the role of self-plasticizer and binder.22 Lynch et al.23 investigated the thermal behavior of brown coals by a proton magnetic resonance thermal analysis and found that about 20% of hydrogen of Victorian brown coal was mobilized under heating to 200 °C even without mechanical pressure, while the coal underwent no softening/fusion in a macroscopic sense. In the present study, the combined effects of the briquetting temperature and pressure on properties of resulting briquettes of Victorian brown coal and coke have been investigated.

2. EXPERIMENTAL SECTION As-mined Loy Yang brown coal (Victoria, Australia) with a moisture content of more than 60 wt % wet was dried in air at ambient temperature until its moisture content decreased to 10 wt % wet, pulverized, and then sieved for collecting a fraction that consisted of particles with sizes smaller than 106 μm, which was employed as the Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 2, 2011 Revised: September 21, 2011 Published: September 21, 2011 296

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Table 1. Distribution of the Particle Size of Loy Yang Coal Based on Results from Sieving range of the particle size (μm)

mass fraction (%)

75
106

9.4

53
75

27.4

38
53

21.6

99.9999 vol %) at a rate of 3 °C/min up to 400, 500, 600, 700, 800, or 900 °C with a holding period of 10 min and then cooled to ambient temperature at an average rate of 100
150 °C/min. The resulting coke was recovered, and its dimensions and mass were measured. The peak temperature for the carbonization will be denoted by TC. The mechanical strength of the coke was measured at ambient temperature by means of diametrical compression tests on a testing apparatus, Shimadzu EZ-L. Four to eight samples prepared under the same conditions were subjected to tests. The displacement and loading were measured during the compression at a displacement rate of 2.00 mm/min.24 The assumption that the maximum loading at the breakage of the specimen corresponded to the maximum tensile stress, Pmax, was determined on the basis of the following equation:

Figure 1. Effect of TB on the yield of resulting coke.

Figure 2. Effects of TB on FB and FC.

temperatures. Figure 1 shows the yield of coke from the carbonization with TC = 900 °C as a function of TB. The briquetting was effective for increasing the coke yield over the entire range of TB. Such an increase was probably due to suppressed diffusion of tar vapor and resultant promotion of its charring inside the pyrolyzing briquette. It was suggested that the briquetting at TB > 100 °C caused additional suppression of tar evolution, implying further changes in the structure of the briquette. The briquette density, FB, was calculated on a moisture-free basis by an equation, FB = (1
w)F, where w and F were the residual moisture content and measured apparent density of the briquette, respectively. This equation is based on an assumption that the briquette undergoes little volumetric contraction in the final stage of moisture release.26 As seen in Figure 2, FB greatly increased from 1.02 to 1.20 g/cm3 at 25
70 °C and further but gradually to 1.26 g/cm3 at 230 °C. According to Higgins et al.27 and Matsuo et al.,28 the true density and micropore volume of dry Loy Yang coal are 1.42 g/cm3 and 0.065 cm3/g, respectively. Assuming neither mesopores nor macropores in the briquette, its density is 1.30 g/cm3. FB of the briquettes prepared at 200
230 °C being as high as 1.24
1.26 cm3/g suggested that the briquetting at such a temperature effectively eliminated interparticle spaces by causing considerable deformation of particles and their adhesion/coalescence and even loss of intraparticle macro-/mesopores. TB influenced the density of coke, FC, in a way slightly different from FB; i.e., there was a minimum of FC. The density of coke was a result from events causing an increase and a decrease in the density of carbonizing briquette. FC had a minimum at TB =

Pmax ¼ 2Lmax =πdl where Lmax, d, and l are the maximum loading, diameter, and thickness of the specimen, respectively. Although not shown in detail, for all of the specimens tested, the loading (stress) increased linearly with the displacement until its sudden drop. The coke samples prepared in the present study were thus broken, obeying a brittle fracture mechanism. It was confirmed that the coke samples cracked nearly equally into two semi-discs, which was a necessary condition in applying the above equation.25 Fractured surfaces as well as top/bottom surfaces of some coke samples were observed by scanning electron microscopy (SEM) on a micrograph (Keyence, VE-9800). Compression strengths were not measured in the present study, because it depends upon the aspect ratio (thickness/diameter ratio) of the briquette.

3. RESULTS AND DISCUSSION 3.1. Effects of the Briquetting Temperature on Properties of Briquette and Coke. Effects of the briquetting temperature,

TB, were examined on the properties of resulting briquette and coke. The briquetting pressure, hereafter referred to as PB, was fixed at 128 MPa. The briquettes that had been prepared at TB = 25, 70, 100, and 130 °C contained 6.6, 4.5, 2.4, and 0.5 wt % moisture, respectively, while less than 0.1 wt % for those at higher 297

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130
160 °C. It was believed that, during the carbonization, FC varied in three events: thermal relaxation of briquette, pore formation because of evolution of volatiles, and densification of the nonporous part of the solid together with shrinkage of pores. No significant effects of applying TB = 130
160 °C was plausible on the second and third events as far as the result shown in Figure 1 was considered. It was therefore suggested that the briquettes that had been prepared at TB = 130
160 °C underwent thermal relaxation, which was associated with volumetric expansion, in the course of reheating. In other words, the coal experienced structural rearrangement during the briquetting at TB > 100 °C in a manner different from that at TB e 100 °C. This hypothesis was consistent with the increase in the coke yield at TB > 100 °C, although the scale of such rearrangement is unknown. Another effect of applying TB > 100 °C on coke property will be shown and discussed later. In Figure 3, the average tensile strength of coke, Pmax, is shown as a function of TB together with standard deviation, σ. Pmax is greater than those of conventional cokes, 2
6 MPa over the entire range of TB studied. It is noted that Pmax steeply increases from 14 to 28 MPa at TB = 100
130 °C. Briquetting at TB > 100 °C was thus effective for preparing high-strength coke with Pmax = 28
37 MPa. 3.2. Effects of the Carbonization Temperature on Properties of Coke. Briquettes were prepared with TB = 200 °C and PB = 128 MPa and carbonized at TC = 400
900 °C for investigating the effects of TC on the property of the resulting coke. The results are summarized in Table 2. Pmax of the briquette, even before carbonization, was as high as 8.5 MPa. Although not shown in this table, Pmax of the briquette with TB = 25 °C and PB = 128 MPa was only 3.5 MPa. The briquetting under heating was thus effective for preparing high-strength briquette. It was believed for the briquette with TB = 25 °C that particles coalesced holding water that acted as a lubricant as well as a plasticizer. There was a

possibility that water evaporated in the early period of carbonization, forming boundaries of particles. On the other hand, it was suggested for the briquette with TB = 200 °C that it was formed obeying a different mechanism of particle coalescence. Direct bonding (i.e., bonding without water) among particles was a possible event in the briquetting in the absence of water. Pmax of coke was not a simple function of TC. It increased from 8.5 to 13.8 MPa at 200
400 °C, remained in a range of 12.3
14.6 MPa at 400
600 °C, and then quickly increased to 36.7 MPa at 600
900 °C. It seemed that an increase in the briquette/coke strength occurred in the early and late periods of pyrolysis, i.e., TC < 400 °C and TC > 600 °C. The relative bulk volume was simply calculated from FC and the coke yield. The briquette/coke continued to shrink over the entire range of TC. No swelling is an indication of no or little softening upon heating in a macroscopic sense. 3.3. Effects of the Briquetting Pressure on Properties of Briquette and Coke. The effects of PB on FB and FC were

Figure 3. Pmax as a function of TB.

Figure 5. Effect of PB on the yield of resulting coke.

Figure 4. Effects of PB on FB and FC.

Table 2. Effect of TC on Properties of Coke from Briquette with PB = 128 MPa and TB = 200 °C TC (°C)

Pmax (MPa)

σ (MPa)

coke yield (wt % dry briquette)

relative bulk volume

a

1.24

8.5

0.9

400

1.11

13.8

0.7

86.1

0.96

500

1.07

12.3

0.6

69.3

0.80

600

1.08

14.6

1.1

62.6

0.73

700

1.10

18.9

1.3

57.4

0.65

800

1.19

27.3

1.1

55.8

0.58

900

1.26

36.7

5.4

54.3

0.53

200

a

FC (g cm
3)

100

1.0

Briquette before carbonization. 298

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the preparation of the coke at high yields over the entire range of PB from 32 to 192 MPa. Figure 6 presents average Pmax as a function of PB. Pmax increased in a similar manner to that of FC, suggesting the importance of FC as a crucial factor for the tensile strength of coke with TC = 900 °C. 3.4. SEM Observation of Coke Samples. Top/bottom and fractured surfaces of cokes prepared under different combinations of TB and PB were observed by SEM. Figure 7 displays SEM photographs of some selected coke samples. One of the features common among the coke samples was the absence or rarity of pores with sizes of 10 μm and greater, and this was a main reason why many of the coke samples had Pmax > 25 MPa. The fractured surface of a coke sample prepared with PB = 128 MPa and TB = 100 °C (see photographs e and f) had morphology very similar to that of another coke from the same PB and TB = 200 °C (see photographs c and d) in terms of the frequency of grain boundaries and size/frequency of pores. On the other hand, the fractured surface of a coke from TB = 200 °C and PB = 32 MPa (see photographs g and h) had clearly more grain boundaries and greater pores than those from PB = 128 MPa. The photographs c
f revealed that the briquetting leading to cokes with FC > 1.2 g/cm3 successfully eliminated grain boundaries. It was found that the briquettes prepared at TB > 100 °C were tinged with black, while the briquettes prepared at TB = 25 °C were dark brown in color. This was an implication of the fact that more or less amount of low-molecular-mass components of the coal was mechanically squeezed out of the macromolecular network, appeared on the particle surface, and promoted its adhesion to another particle. It was also implied that such low-molecular-mass components, even inside the particle matrix, played the role of plasticizer, enhancing softening and deformation of particles. Some briquettes were crushed and then subjected to extraction with tetrahydrofuran (THF) at 30 °C and under ultrasonic irradiation. THF extracted material with 12.9 wt % of the briquette. This extraction yield was nearly equivalent to that from the starting coal, 13.6 wt %. The result was explained by the fact that the mechanical extraction of lowmolecular-mass components, if occurred, was not so extensive as the extraction with THF. Roles of low-molecular-mass components were further investigated. The coal was extracted with a mixture of THF and methanol (7:3 in volume; Hildebrand solubility parameter = 11.9 cal0.5 cm
1.5) under ultrasonic irradiation and at ambient temperature. The choice of these mixed solvents was based on knowledge given in a previous report,29 which showed that the extraction yield from another Victorian brown coal was maximized by adjusting the Hildebrand solubility parameter around 12 cal0.5 cm
1.5. The extraction was not performed exhaustively, but the mixed solvents extracted a 14.5% mass fraction of the coal. The extraction residue, after complete removal of the solvents, was briquetted with TB = 200 °C and PB = 128 MPa and then carbonized at TC = 900 °C. On the other hand, the extract was mixed with the coal at a mass ratio of 10:90 and then subjected to briquetting and carbonization in the same manner as for the extraction residue. Table 3 compares properties of the briquettes and cokes from the different materials. Removal of the extractable material resulted in the reduction of Pmax by about 10 MPa. Assuming that the extractable material represented a major portion of low-molecular-mass components of the coal, this reduction indicated that such components played the role of plasticizer and/or self-binder in the briquetting and subsequent carbonization. However, it should also be noted that the coke from

Figure 6. Pmax as a function of PB.

Figure 7. SEM images of fractured surfaces (a and c
h) and top/ bottom surface (b) of coke samples. Images a, c, and d: TB = 200 °C and PB = 128 MPa. Images b, e and f: TB = 100 °C and PB = 128 MPa. Images g and h: TB = 200 °C and PB = 32 MPa.

investigated with TB fixed at 200 °C. As seen in Figure 4, both FB and FC increased monotonously with PB in a very similar manner to each other. FB and FC were very close to each other, but this was just a result for TC = 900 °C. According to the data shown in Table 2, different TC values gave different FB
FC relationships. Figure 5 illustrates the effect of PB on the coke yield. In comparison to the effect of applying TB = 200 °C on the coke yield (see Figure 1), the effect of increasing PB was less significant. In other words, briquetting at TB = 200 °C enabled 299

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Table 3. Effects of the Addition and Removal of THF/Methanol-Extractable Material on Properties of Briquette and Coke FB (g cm
3)

FC (g cm
3)

Pmax (MPa)

σ (MPa)

coke yield (wt % briquette)

coal

1.25

1.26

36.7

5.4

54.3

extraction residue

1.22

1.21

26.8

2.6

55.0

extract + coal

1.25

1.23

32.1

1.2

54.2

material

believed that such high density, in other words, low porosity, was the primary reason for the very high tensile strength of the cokes. The polished surface of a coke sample (TB = 200 °C and PB = 128 MPa) was observed by optical polarized-light microscopy, which confirmed no formation of optically anisotropic textures during the carbonization. A thermomechanical analysis of a briquette (TB = 200 °C and PB = 128 MPa) in a needle penetration mode detected shrinkage but no softening/fusion of the briquette upon heating to 600 °C. These results are proof that the high strength of the present cokes was not due to carbonization via mesophase formation, requiring fusion of the coal matrix upon heating. Recently, Benk et al.32,33 reported the production of briquettes having a tensile strength of over 50 MPa from coke breeze with phenolic resin binders (resol and novalac binders). The binders behave as thermosets after curing; in other words, these have no fusibility during carbonization. This is an example demonstrating that the occurrence of thermoplasticity in a temperature range of pyrolysis is not a necessary condition for the production of highstrength coke. The brown coal used in this work is a thermoset, but it can be converted to a high-strength coke if adhesion and coalescence of feedstock particles have occurred to a sufficient degree prior to carbonization. Discussed here is the presence of two different FC
Pmax relationships for the briquetting at TB > 100 °C and that at TB e 100 °C. As shown in some photographs of Figure 7, it was difficult to explain the difference in the fractured surface between the cokes from briquettes with TB = 100 and 200 °C. The briquettes prepared at TB e 100 °C contained 2.4
6.6 wt % moisture, which played the role of plasticizer and/or lubricant in the briquetting, helping the formation of briquettes with FB as high as 1.2 g/cm3. It was believed that low-molecular-mass components of the coal did not play important roles because of the insufficiently high temperature for breaking hydrogen bonds and, in addition, the presence of water that was not necessarily a good solvent of such components. On the other hand, the briquetting at TB > 100 °C effectively removed water to a content below 0.5 wt % and allowed for the mobilization of the entire part of the coal. As shown in Figure 2, the densities of the briquettes with TB = 130 and 160 °C decreased upon the reheating (for the carbonization) to degrees much greater than those with TB e 100 °C. This was probably caused by thermal relaxation (volumetric expansion) of the briquettes in the early stage of carbonization. Such thermal relaxation might release briquetting-induced mechanical stress remaining inside the briquettes that could induce defects, such as microcracks, during the carbonization. Thus, the thermal relaxation, if occurred, played the role of annealing of the briquette. For the briquetting at TB = 200
230 °C, the temperature would be high enough for the annealing during briquetting. It was estimated that the resulting briquette would be carbonized with less extensive volumetric expansion.

Figure 8. Relationships between FC and Pmax.

the extraction residue had Pmax as high as 27 MPa. It seemed that the coal experienced softening and deformation under 128 MPa pressure at 200 °C even in the absence of such low-molecular-mass components, as represented by the extractable material. Table 3 also reveals that that addition of the extractable material was not effective on the further increase in Pmax of the resulting coke. It was estimated that plasticization of the extractable material occurred more easily than the entire portion of the coal and that the material underwent fusion in the course of carbonization.23 The strength of the coke from the mixture of the coal and extractable material was, nonetheless, even lower than that from the coal alone. The reason for such a negative effect of the addition of the extractable material is unknown, but it could at least be said that adhesion and coalescence among coal particles occurred to a sufficient degree in the briquetting with TB = 200 °C and PB = 128 MPa without a binder. 3.5. Factors Crucial to the Mechanical Strength of Coke. Figure 8 summarizes combined effects of TB and PB on the average Pmax, hypothesizing that FC is a critical property for the mechanical strength of coke. Two different relationships between Pmax and FC are seen in the figure. Pmax of the coke from briquetting at TB > 100 °C is a linear function of FC over the ranges of FC = 0.87
1.27 cm3/g and PB = 32
192 MPa. In the case of TB e 100 °C, Pmax is described by another linear function of FC. Briquetting with TB > 100 °C was thus found to be necessary for preparing coke with Pmax > 15 MPa within the conditions examined. Each of the linear relationships is explained as follows. Under an assumption that all of the cokes prepared in this work had similar true densities,29 the difference in FC among cokes from different conditions is mainly attributed to that in the porosity. Patrick and Stacey30 showed that the tensile strength of coke was welldescribed as a function of porosity. Arima31 claimed that the strength of the nonporous part of coke was not necessarily a strong function of the coal rank/type and fusibility, while frequencies of connected pores and non-adhesion grain boundaries were more crucial because mechanical stress tends to be concentrated there. FC of the present cokes, ranging from 0.87 to 1.27 cm3/g, are much higher than general blast furnace and foundry cokes. It was thus

4. CONCLUSION Briquettes from pulverized Loy Yang brown coal were prepared with mechanical pressures of 32
192 MPa and temperatures 300

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of 25
230 °C but without a binder and carbonized by heating to 900 °C without mechanical pressure. The resulting cokes had tensile strengths of 6
37 MPa. Such high strength was mainly attributed to the low porosity of coke, which originated from that of the briquette. Briquetting at temperatures over 100 °C caused thermomechanical plasticization/deformation of the entire part of coal, thereby promoting coalescence/adhesion of particles and eliminating interparticle spaces and grain boundaries, while a lowmolecular-mass component enhanced the plasticization to some degree.

(21) Miura, K.; Mae, K.; Sakurada, K.; Hashimoto, K. Energy Fuels 1992, 6, 16–21. (22) Allardice, D. J.; Chaffee, A. L.; Jackson, W. R.; Marshall, M. In Advances in Science of Victorian Brown Coal; Li, C.-Z., Ed.; Elsevier: Amsterdam, The Netherlands, 2004; Chapter 3. (23) Lynch, L. J.; Sakurovs, R.; Webster, D. S.; Redlich, P. J. Fuel 1988, 67, 1036–1041. (24) Yamazaki, Y.; Hayashizaki, H.; Ueoka, K.; Hiraki, K.; Matsushita, Y.; Aoki, H.; Miura, T. Tetsu to Hagane 2004, 90, 536–544. (25) Mellor, M.; Hawkes, I. Eng. Geol. 1971, 5, 173–225. (26) Evans, D. G. Fuel 1973, 52, 186–190. (27) Higgins, R. S.; Kiss, L. T.; Allardice, D. J.; George, A. M.; King, T. N. W. State Electricity Commission of Victoria (SECV) Research and Development Department Report; SECV: Melbourne, Victoria, Australia, 1980, Report SC/80/17. (28) Matsuo, Y.; Hayashi, J.-i.; Kusakabe, K.; Morooka, S. Coal Sci. Technol. 1995, 24, 929–932. (29) Hayashi, J.-i.; Aizawa, S.; Kumagai, H.; Chiba, T.; Morooka, S. Energy Fuels 1999, 13, 69–76. (30) Patrick, J. W.; Stacey, A. E. Fuel 1978, 57, 258–264. (31) Arima, T. Tetsu to Hagane 2001, 87, 274–281. (32) Benk, A.; Talu, M.; Coban, A. Fuel Process. Technol. 2008, 89, 28–37. (33) Benk, A.; Talu, M.; Coban, A. Fuel Process. Technol. 2008, 89, 28–46.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: +81-92-583-7796. Fax: +81-92-583-7793. E-mail: [email protected].

’ ACKNOWLEDGMENT This study was carried out as a part of a research project, “Scientific Platform of Innovative Technologies for Co-upgrading of Brown Coal and Biomass”, which has been financially supported by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan, in a Program of Strategic Funds for the Promotion of Science and Technology. The authors are also grateful to The Iron and Steel Institute, Japan (ISIJ), and the Nippon Steel Corporation (NSC) for financial support. Prof. Tsuyoshi Hirajima, School of Engineering, Kyushu University, and Dr. Seiji Nomura, NSC, are acknowledged for their technical advice.

’ NOTE ADDED AFTER ASAP PUBLICATION The second paragraph of the Introduction section was modified, and an additional reference added to the version of this paper published October 5, 2011. The correct version published October 17, 2011.

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