Preparation of Coke from Indonesian Lignites by a Sequence of

Oct 28, 2013 - Research Centre for Geotechnology, Indonesian Institute of Sciences (LIPI), Jl. Sangkuriang Komplek LIPI, Gd. 70, Bandung 40135,. Indon...
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Preparation of Coke from Indonesian Lignites by a Sequence of Hydrothermal Treatment, Hot Briquetting, and Carbonization Aska Mori,† Mutia Dewi Yuniati,‡ Anggoro Tri Mursito,§ Shinji Kudo,∥ Koyo Norinaga,† Moriyasu Nonaka,‡ Tsuyoshi Hirajima,‡,∥ Hyun-Seok Kim,∥ and Jun-ichiro Hayashi*,†,∥ †

Institute for Materials Chemistry and Engineering, Kyushu University, 6-1, Kasuga Koen, Kasuga 816-8580, Japan Department of Earth Resources Engineering, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § Research Centre for Geotechnology, Indonesian Institute of Sciences (LIPI), Jl. Sangkuriang Komplek LIPI, Gd. 70, Bandung 40135, Indonesia ∥ Research and Education Center of Carbon Resources, Kyushu University, 6-1, Kasuga Koen, Kasuga 816-8580, Japan ‡

ABSTRACT: Production of coke from lignites was studied in continuation of a previous study that demonstrated effectiveness of a sequence of hot briquetting and carbonization on preparation of high strength coke from a lignite. Cokes were prepared from four Indonesian lignites with or without pretreatments such as hydrothermal treatment (HT) at 200−300 °C, acid washing (AW), and a combination of them (HT−AW). The hot briquetting of the raw lignites at temperature and mechanical pressure of 200 °C and 128 MPa, respectively, enabled cokes to be produced with a tensile strength (TS) of 7−22 MPa. The pretreatments, AW and HT at 200 °C (HT200), increased TSs of resulting cokes to 18−24 and 13−36 MPa, respectively. A sequence of HT200 and AW further increased TSs of cokes to 27−40 MPa. AW and HT200 modified the macromolecular structure of the lignites by different mechanisms. AW removed alkali and alkaline earth metallic species that played roles of cross-links in the macromolecular network, while HT200 rearranged macromolecules physically. Both HT and AW enhanced plasticization and then deformation/coalescence of lignite particles during the briquetting, which formed high strength briquettes. There were strong correlations between TS of coke and that of briquette and also between TS and bulk density of coke from the individual lignites. temperature above 100 °C successfully caused thermomechanical plasticization of the entire part of the lignite matrix inducing particles’ deformation/coalescence, eliminating intraparticle spaces and grain boundaries. Cold briquetting of Loy Yang coal enabled coke to be produced,10 but it is strength and bulk densities were clearly lower than those from the hot briquetting above 100 °C. It was also found that residual moisture of the lignite had negative impacts on the strength and bulk density of the resulting coke,10 which was a reason why the briquetting at a temperature over 100 °C was more effective than that at a lower temperature. Thus, in application of the hot briquetting to practical coke production, a pretreatment, that is, drying is preferable or essential prior to briquetting. Drying is the primary process in any modes of lignite utilization, and a number of drying processes have therefore been proposed.11 Heat treatment of lignite in hot compressed water, which is generally called hydrothermal dewatering or drying (HTD), is a type of nonevaporative drying, and it has an advantage over evaporative drying because it theoretically enables latent heat of water to be saved although the process must be operated at a pressure higher than saturated water vapor pressure at 180−350 °C.11 HTD is a solvent treatment at a temperature where thermochemical reactions take place, and

1. INTRODUCTION Production of metallurgical and foundry cokes from lignite as well as sub-bituminous coal is an important option for sustainable metallurgical industries. Production of high quality coke from such lower rank coal is effective in reducing the consumption of coking coal and also in realizing iron-making processes with more blast furnace reaction efficiency, which needs coke with high reactivity and strength.1,2 It is generally agreed that coke from lower rank coal has a higher reactivity with CO2 than that from coking coal3 mainly due to the presence of metallic species that catalyze the Boudoir reaction (C + CO2 = 2CO) and a well developed porous structure. Thus, the production of high strength coke from lower rank coal has significance in the future of making iron. There have been studies on binderless briquetting of lower rank coals before carbonization to form coke4−10 such as cold briquetting of lignites6 and sub-bituminous coal7 and hot briquetting of a single sub-bituminous or bituminous coal8 or its blend with coking coal9 at 300−500 °C. The present authors10 recently studied binderless briquetting of pulverized Victorian lignite (Loy Yang) at temperatures of 130−230 °C and mechanical pressures of 32−192 MPa, which enabled cokes with tensile strength of 6−37 MPa with a carbonization temperature of 900 °C to be produced. Such tensile strength, much higher than those of conventional metallurgical cokes, arose from a bulk density as high as 1.2−1.3 cm3/g, in other words, low porosity of the cokes. The briquetting at a © 2013 American Chemical Society

Received: August 19, 2013 Revised: October 27, 2013 Published: October 28, 2013 6607

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Table 1. Properties of Raw and Treated Lignite Samples before Briquetting lignite ID

treatment ID

A

raw AW HT200 HT250 HT300 HT200-AW raw AW HT200 HT250 HT300 HT200-AW raw AW HT200 HT250 HT300 HT200-AW raw AW HT200 HT250 HT300 HT200-AW treatment ID

B

C

D

lignite ID A

B

C

D

raw AW HT200 HT250 HT300 HT200-AW raw AW HT200 HT250 HT300 HT200-AW raw AW HT200 HT250 HT300 HT200-AW raw AW HT200 HT250 HT300 HT200-AW

moisture wt %

C wt %-daf

9.5 65.7 8.8 67.4 8.3 67.6 6.0 67.5 8.7 72.4 11.0 66.6 9.0 65.0 11.3 66.5 8.7 66.4 6.3 68.2 6.3 71.5 7.6 66.0 12.8 66.2 10.2 67.9 10.2 68.3 7.2 68.3 8.6 72.0 8.7 67.4 11.6 68.1 15.1 68.4 10.2 68.5 5.8 69.1 7.2 72.2 9.4 66.8 yield wt %-dry-raw-lignite 1 0.97 0.97 0.97 0.92 0.94 1.00 0.97 0.98 0.95 0.87 0.96 1.00 0.99 0.98 0.97 0.93 0.96 1.00 0.97 0.99 0.98 0.92 0.96

H wt %-daf

O + S wt %-daf

4.8 4.4 4.5 4.7 4.6 4.6 5.4 5.1 5.2 5.4 5.4 5.3 4.7 4.4 4.4 4.6 4.4 4.6 4.6 4.5 4.5 4.8 4.7 4.7

28.6 27.3 27.0 26.8 22.0 28.0 28.6 27.4 27.4 25.4 22.1 27.7 28.2 26.9 26.4 26.1 22.6 27.1 26.3 26.0 26.0 25.1 22.0 27.6

N wt %-daf

H/C ratio

O/C ratio

0.9 0.8 0.9 0.9 1.0 0.9 1.0 1.0 1.0 1.0 1.0 1.0 0.8 0.9 0.9 0.9 1.0 0.9 1.0 1.0 1.0 1.1 1.1 1.0 ash wt %-dry-raw-lignite

0.86 0.78 0.79 0.84 0.76 0.82 0.99 0.92 0.93 0.94 0.89 0.95 0.85 0.77 0.77 0.80 0.73 0.81 0.81 0.79 0.79 0.82 0.78 0.83

0.33 0.30 0.30 0.30 0.23 0.32 0.33 0.31 0.31 0.28 0.23 0.31 0.32 0.30 0.29 0.29 0.24 0.30 0.29 0.29 0.29 0.27 0.23 0.31

3.2 0.6 3.1 3.1 3.3 0.2 6.8 4.0 6.6 6.6 6.5 4.5 1.8 0.4 2.0 1.8 1.8 0.2 3.1 0.0 3.1 2.7 2.5 0.0

then it induces chemical as well as physical changes in the macromolecular structure of lignite. A decrease in the oxygen content is a particular feature of HTD, and it contributes to reduction of equilibrium moisture content and increase in the calorific value on a dry basis. It is also known that HTD extracts inorganic constituents such as sodium, magnesium, and calcium, i.e., alkali and alkaline earth metallic species, that are present in the lignite in a form of organically bound cation or inorganic salt.11,12 Favas et al.12 studied HTD of Victorian lignites under mechanical pressure, which is called mechanical

ash wt %-dry 3.2 0.7 3.2 3.2 3.5 0.2 6.8 4.1 6.8 7.0 7.5 4.7 1.8 0.4 2.0 1.8 1.9 0.2 3.1 0.0 3.1 2.8 2.8 0.0

thermal expression (MTE), and found that inorganic salts dissolved in the pore water of the raw lignites were removed entirely by MTE. It was also reported that removal of sodium, which was a major inorganic species in Victorian lignites, increased linearly with increasing removal of water.13 When a combination of HTD and the hot briquetting/ carbonization is considered for production of coke from lignite, physicochemical changes of the lignite by HTD and how such changes influence the behavior of the lignite during the hot briquetting and subsequent carbonization need to be known. In 6608

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Figure 1. Tensile strength (TS) of coke. at 60 °C with a solution/lignite mass ratio of 30:1. After 8 h treatment, the solid was washed with distilled water until no chlorine ion was detected by an AgNO3 titration method and then dried until the moisture content reached 8−12 wt %-wet. 2.4. Hot Briquetting and Carbonization. The lignite samples were briquetted according to the same procedure as reported previously.10 In brief, about 1 g of the sample was transferred into a 14.1 mm diameter mold and heated to a prescribed temperature of 200 °C. Then, a mechanical pressure of 128 MPa was applied by hydraulic loading for 8 min. The pressure was then released, and the resulting briquette was recovered, while the mold was cooled naturally to ambient temperature. The dimensions, mass, and moisture content of the briquette in a form of disc were measured. The typical diameter and thickness of the briquette were 14.0 and 5.0 mm, respectively. Coke was prepared by heating the briquette in a horizontal quartzmade tubular reactor in atmospheric flow of nitrogen (purity >99.9999 vol. %) at a rate of 3 °C/min up to 900 °C with a holding period of 10 min, which was followed by cooling to ambient temperature at an average rate of 100−150 °C/min. The resulting coke was recovered, and its dimensions and mass were measured. 2.5. Measurement of Mechanical Strength of Briquette and Coke. The mechanical strengths of the briquettes (before carbonization) and cokes were 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.16 The assumption that the maximum loading at the breakage of the specimen corresponded to the maximum tensile stress, TS, was determined on the basis of the following equation:

the present study, in continuation of the authors’ previous study,10 effects of the hydrothermal treatment on properties of briquette and coke such as bulk density and mechanical strength were investigated employing Indonesian lignites.

2. EXPERIMENTAL SECTION 2.1. Lignite Samples. Four Indonesian lignites from different mines (A, B, C, and D) were employed as the original samples. Table 1 lists the ultimate analysis of the lignites. Every as-received lignite sample with a moisture content of 40−60 wt %-wet and particle sizes smaller than 40 US mesh was subjected to hydrothermal treatments without predrying. Raw and treated coal samples were dried to reduce moisture contents to 8−12 wt %-wet at ambient temperature and pulverized to sizes below 140 US mesh prior to hot briquetting or acid washing. The partially dried and pulverized lignite is hereafter termed “raw lignite” for expedience. 2.2. Hydrothermal Treatment. The as-received lignite sample (40 g) and distilled water (260 g) were charged into a 0.5 L batch autoclave (Taiatsu Techno, Type MA22), which was then pressurized to 2.0 MPa with nitrogen (purity >99.999 vol. %), heated to a peak temperature of 200, 250, or 300 °C at a rate of 6.5 °C/min. The peak temperature was held for 30 min while the slurry was continuously stirred at a rate of 200 rpm. After the treatment, the autoclave was quenched to ambient temperature and opened. The resulting slurry was filtered with filter paper (Advantec, Type C). A detailed procedure is reported elsewhere.14,15 The equilibrium moisture contents of the resulting solids decreased as the treatment temperature was raised. For all of the lignites A−D, the equilibrium moisture contents, were in the following ranges: 200 °C; 13−14 wt %-wet, 250 °C; 7−9 wt %-wet, 300 °C; 4−6 wt %-wet. The resulting solid was dried at ambient temperature and under vacuum until the moisture content decreased to about 10 wt %-wet, pulverized to sizes smaller than 140 mesh, and then subjected to hot briquetting or acid washing. The hydrothermal treatment at 200, 250, and 300 °C will be denoted by HT200, HT250, and HT300, respectively. A lignite sample from hydrothermal treatment will be indicated with an abbreviation such as “HT200 lignite A” and “HT250 lignite B”. 2.3. Acid Washing. The raw and hydrothermally treated lignites were treated in an aqueous solution of 3 N hydrogen chloride (HCl)

TS =

2Lmax πdl

Lmax, d, and l are the maximum loading, diameter, and thickness of the specimen, respectively. Details of measurements are reported elsewhere.16 Fractured surfaces of the coke samples were observed by scanning electron microscopy (SEM) on a micrograph (Keyence, VE9800). Compression strengths were not measured in the present study, because it depends upon the aspect ratio (thickness/diameter ratio) of the briquette/coke. The mechanical strength of coke is more 6609

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generally evaluated based on tumble drum indices (DI), but such properties were not measured in the present study due to difficulty of preparing enough number/amount of specimens. It is, however, known that TS and DI are normally correlated well with each other.17 2.6. Solvent Extraction. The contents of solvent-extractable matter in the raw, acid washed, and hydrothermally treated lignites were investigated by subjecting them to extraction with a solvent, tetrahydrofuran (THF, a reagent grade), at 40 °C under ultrasonic irradiation. A 320−360 mg portion of the sample was used for the extraction with 40 mL of fresh solvent. After 2 h of extraction, the slurry was separated into the solid and liquid by centrifugation at 3,500 rpm for 60 min. Then the solid was mixed with another 40 mL of fresh solvent and further extracted. The extraction in such a way was repeated three times. The remaining solid, i.e., extraction residue, was washed with 40 mL of a reagent grade methanol and finally dried at 50 °C for 18 h under vacuum. The extractable matter was recovered by evaporating the solvents, and it was further dried in the same way as the extraction residue. The total mass of the extractable matter and residue was slightly more than that of the sample before the extraction on a dry basis, 102−103%, probably due to incomplete desorption of THF. The extraction yield was then defined by

the plasticization during the briquetting. It is also agreed that the removal of AAEM species promotes thermal degradation of the macromolecules enhancing the plasticity during the coal pyrolysis.25 In the present study, either softening or fusion of the briquettes was not significant as to cause their macroscopic deformation (except shrinkage) and swelling in bulk. However, even so, it was plausible that extensive thermal degradation enhanced particles’ coalescence during the pyrolysis. Yip et al.5 studied the pyrolysis and carbonization of briquettes from a sub-bituminous coal and found an important role of tar to fill interstitial space among particles in the briquette during the pyrolysis. Significant promotion of tar formation by AW has been proven by many previous studies on the pyrolysis of lignite.23−26,28−31 The effects of AW on the briquette/coke properties will be discussed in more detail later. Figure 1 also demonstrates positive effects of HT, in particular, HT200. This treatment increased TS of the coke from all lignites A−D. Increases in TSs of the cokes from lignites B and C were even greater than those by AW. It has been reported that hydrothermal treatment can remove a portion of AAEM species,12,13 but this was not the case of HT under the present experimental conditions. As shown in Table 1, there was no or little reduction of the ash content by HT regardless of the treatment temperature. Moreover, HT200 caused neither noticeable change in the elemental composition nor extraction of organic matter. It was thus suggested that HT200 had modified the physical structure of the organic matrix of the lignites so that these experienced more extensive plasticization during the briquetting and/or the subsequent carbonization. Further increase in the HT temperature to 250− 300 °C seemed to give a negative effect on TS. More decrease in the O/C ratio by HT250 and HT300 than by HT200 resulted from more extensive progress of thermochemical reactions removing oxygen-containing functionality, which was unfavorable for the occurrence of plasticity during the briquetting. It is known that dehydration condensation between oxygen containing functional groups such as hydroxyls and carboxyls, which is a major contributor to the cross-linking, begins even at temperatures as low as 200 °C upon heating of lignite.29 Even so, the cokes from HT250 lignites had TSs higher than or equivalent to those from the original lignites. AW of HT200 lignite removed AAEM species as extensively as that of the raw lignite. TS of coke from HT200-AW lignites ranged from 27−40 MPa, and more importantly, it was greater than those from not only HT200 but also AW lignites. This result, common among the four lignites, indicated that both AW and HT contributed to an increase in TS of coke but by different mechanisms, which were removal of AAEM species and physical change in the macromolecular structure, respectively. 3.2. Properties of Coke/Briquette Crucial for Strength of Coke. TS of coke is plotted against its bulk density in Figure 2. For every lignite, TS of coke was strongly related to its bulk density. Linear relationships between TS and the bulk density were in broad agreement with that reported for cokes prepared from a Victorian lignite that were hot briquetted under a variety of mechanical pressure/temperature combinations.8 Importance of the bulk density, in other words, that of porosity, has been reported by researchers,32,33 who found that the frequency of connected pores or nonadhesion grain boundaries was crucial for the mechanical strength of coke. The TS−bulk density relationships shown in Figure 2 demonstrated that HT200, AW, and AW-HT200 led to densification of the coke

extraction yield = (mass of exatractable matter) /[(mass of exatractable matter) + (mass of exatractable residue)] 2.7. DSC Analysis. Thermal properties of the raw, AW and HT200 lignites were investigated by a differential scanning calorimetry (DSC) in a range of temperature relevant to the briquetting, i.e., up to 200 °C. A Netzsch DSC 204 F1 Phoenix model was used for the analysis. Each sample was heated or cooled in atmospheric N2 flow (flow rate; 100 mL/min) in the following sequence; (1) heating from ambient temperature to 110 °C at a rate of 20 °C/min, (2) heating at 110 °C for 90 min, (3) natural cooling to 30 °C in 90 min, (4) heating from 30 to 200 °C at 10 °C/min, (5) natural cooling to ambient temperature.

3. RESULTS AND DISCUSSION 3.1. Effects of Acid Washing and Hydrothermal Treatment on Strength of Resulting Coke. Figure 1 shows TSs of cokes prepared from the raw, acid washed (AW), and hydrothermally treated (HT) lignites. The cokes from the raw lignites had TS in a range from 6.6 to 22.1 MPa, which was higher than those of conventional metallurgical cokes produced in coke ovens, 2−6 MPa.18−22 This confirmed the effectiveness of the hot briquetting at 200 °C in preparation of high-strength coke from lignite, which was previously claimed by the present authors.8 AW caused more or less increase in TS of the coke from all of the lignites examined. TSs of cokes from the lignites A and D were increased by 2.7−2.8 times by AW. As shown in Table 1, this treatment decreased the ash contents of the lignites A and D from 3.2 to 0.7 and from 3.1 to 0.0 wt %-dry, respectively. It was believed that such decreases were mainly due to removal of alkali and alkaline earth metallic (AAEM) species that had been present in the lignites in forms of organically bound cations and inorganic salts such as NaCl.23,24 Wornat and Sakurovs25 found that thermally and thermochemically induced mobility of macromolecular network of a Victorian lignite was hindered by divalent cations such as Ca2+ and Mg2+ and also by monovalent Na+. It is widely accepted that the divalent cations behave as cross-links in the macromolecular network of coal.23,26,27 In preparation of the briquette, the hot briquetting caused deformation and coalescence of lignite particles, both of which were attributed to thermomechanically induced plasticization of the macromolecular network.10 AW thus enhanced 6610

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that the flat/smooth surfaces had been formed at the breakage of the coke while the rough surfaces had been present as inner surfaces (pore surfaces) before the breakage. For the lignites A, B, and D, the flat/smooth surface of the cokes from AW and HT200 lignites seemed to be more frequent than that from the raw lignites. It was also found for lignite B that the fractured surface of the coke from HT200 lignite was more smooth and flatter than that that from AW lignite. These results of SEM observation were consistent with those shown in Figure 2, assuming that the coke with higher TS and higher bulk density (i.e., lower porosity) was broken leaving a flat/smooth surface with more frequency and/or greater area on the fractured surface. In the case of lignite C, there was no significant difference in the surface morphology between the cokes from the raw and AW lignites, while the surface of the coke from HT200 lignite was clearly smoother than the other two. This result for lignite C was consistent with those shown in Figures 1 and 2. The bulk densities and TSs of briquettes before carbonization were also measured. Figure 4 shows the relationships between TS of coke and that of briquette. Strong correlations are seen for the raw and hydrothermally treated lignites: HT200, HT250, HT300, and HT200-AW, as indicated by straight lines in the figure. Preparation of a high strength briquette was thus a requirement for producing a high strength coke. It is also seen in the figure that the briquette from AW lignite gave a coke possessing TS clearly greater than that predicted by the straight line. That is, AW lignite was molded to a briquette with TS equivalent with or even lower than that from the raw lignite but yielded a coke with greater TS. As mentioned previously, AW removed AAEM species from the lignites, promoting the thermal degradation and tar formation during the pyrolysis, thereby enhancing coalescence of particles and/or filling of interparticle space, i.e., pores. More extensive

Figure 2. Relationship between bulk density and TS of coke.

from all four lignites. It is also seen that a temperature over 250 °C was out of the optimum range of conditions for the hydrothermal treatment. Figure 3 shows SEM photographs of fractured surfaces of the cokes from the raw, AW and HT200 lignites. The fractured surfaces were formed at the breakage of the coke in the measurement of TS. Two different surfaces, i.e., flat/smooth and rough ones, are seen in every photograph. It was deemed

Figure 3. SEM images of fractured surface of coke. 6611

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Table 2. Coke Yield from Briquette lignite ID

treatment ID

yield wt %-dry-briquette

A

raw AW HT200 HT250 HT300 HT200-AW raw AW HT200 HT250 HT300 HT200-AW raw AW HT200 HT250 HT300 HT200-AW raw AW HT200 HT250 HT300 HT200-AW

54.9 54.9 56.1 56.7 59.5 55.5 53.6 56.7 55.5 56.4 59.2 54.1 57.3 56.9 58.9 59.2 62.6 57.8 54.6 54.8 55.8 56.9 59.1 55.8

B

C

D

Figure 4. Relationship between TS of coke and TS of briquette.

thermal degradation and tar formation during the carbonization thus led to an increase in TS of the coke. As already shown in Figure 4, the briquetting of HT200 lignites gave briquettes with TS greater than those from the raw lignites. This strongly suggested that more significant plasticization and coalescence of particles occurred during the briquetting of HT200 lignites than that of the raw lignites. Table 2 compares the mass yield of coke based on the mass of briquette between raw and HT200 lignites. It is seen that HT200 slightly but systematically increases the coke yield. This trend is attributed to not difference in the chemical structure/ composition between the briquettes from the raw and HT200 lignites but to that in the physical structure. According to the data shown in Figure 4, the briquette from the HT200 lignite is deemed to be less macroporous than that from the raw lignite. Such a nature can suppress transport of tar precursors inside the pyrolyzing briquette, decreasing the tar yield and increasing the coke yield. More significant conversion of the tar precursors leads to filling up of more volume of macropores in the briquette.7 It is difficult to obtain a direct proof of this type of phenomenon, which is at least consistent with the results shown in Figures 3 and 4. As shown in Figure 2, the density of coke from the HT200 lignite is higher than that from the raw lignite in spite of the fact that the briquette of the former (before the carbonization) has a lower density than that of the latter. The density of the briquette will be discussed later. Figure 5 compares the THF extraction yields from the raw, AW, and HT lignites before briquetting. AW increased the extraction yield, and this was undoubtedly due to removal of the AAEM species that had been forming cross-links in the macromolecular structure of the lignite.34,35 On the other hand, the extraction yields from HT200 lignites were lower by 0−3

Figure 5. Mass fraction of THF-extractable matter.

wt % than those from the raw lignites. Such lower yields were explained by extraction of a small portion the organic matter into the water during HT200 (see solid yields of HT200 lignites shown in Table 2). More importantly, HT200 formed no or, if any, very little THF-extractable matter. This fact, taken together with clearly higher TS of the briquette from HT200 lignite than that from the raw lignite, indicated that the mass fraction of the THF-extractable matter was not a measure for extent of plasticization during the briquetting. TS of the briquette from HT200 lignite was higher than that from AW lignite in the cases of lignites B and C, while the extraction yields from HT200 lignites were lower than those from AW lignites. Thus, there was no evidence that the THF-extractable matter played the important role of either plasticizer or binder of particles during the briquetting. It was rather suggested that the hot briquetting under mechanical pressure caused plasticization of the entire or major portion of the organic matter of the lignite.7 6612

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Figure 6. Relationship between bulk density and TS of briquette.

Figure 7. Changes in specific heats of raw, AW, and HT200 lignites with temperature upon heating in inert atmosphere at 10 °C/min.

The extraction yields from HT300 lignites were higher than those from HT200 and HT250 lignites. During HT300, the hydrothermal pyrolysis occurred forming THF-extractable low molecular-mass components, but the pyrolysis also involved the cross-linking, by which plasticizability of the lignite was partly lost. Again, the THF extraction yield was not an important property for the plasticizability during the briquetting. 3.3. Structural Change of Lignite during HT. TSs of briquettes are plotted against their density in Figure 6. Different

from TS of the coke, that of the briquette was not simply related to its density. AW increased the briquette density but not its TS. The increase in the density was reasonably explained by more extensive plasticization due to removal of AAEM species. On the other hand, a slight decrease in TS of the briquette resulting from AW for lignites A, B, and C appeared to be inconsistent with the increased density. The main reason for such inconsistency was unknown, but a possible explanation was residual stress in the briquette. A high degree of 6613

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Figure 8. Scheme of improvement of TS of coke by HT200.

plasticization of the AW lignite enabled more extensive densification of the briquette, while the excess densification induced residual stress in the resulting briquette and then caused TS decrease. It is noted in Figure 6 that the briquette from HT200 lignite had a clearly lower bulk density but higher TS than that from the raw lignite. The increase in TS of the briquette by HT200 was probably due to enhanced plasticization and particles’ coalescence, but these were not a reason for the decrease in the bulk density. Taken together with very little change in the elemental/chemical composition by HT200, it was believed that the decrease in the bulk density arose from that in the density at a scale much smaller than that of particles. It was speculated that such structural change was induced by physical rearrangement of macromolecules causing the density decrease. The decrease in the density at a molecular or macromolecular scale will be discussed later. In HT200, water at 200 °C played a role of solvent that swelled the lignite solvating polar functionalities such as hydroxyl, carboxyl, and carbonyl groups. It was then probable that the molecular arrangement after HT200 was physically different from that in the raw lignite. Such difference was acceptable from the property of HT200-AW lignite. The briquette from HT200-AW lignite had a bulk density as high as that from the raw lignite, but much higher TS. This was explained well by physical rearrangement of the macromolecular structure by HT200 and subsequent removal of AAEM species by AW. Increasing temperature for the hydrothermal treatment from 200 to 250−300 °C decreased both TS and the bulk density of the briquette. The decrease in the bulk density was mainly due to the decrease in the oxygen content as well as the degree of plasticization, which was brought about by cross-linking reactions such as dehydration during HT. The results of DSC are presented in Figure 7. Specific heats of the raw, AW, and HT200 lignites are compared at 50−200 °C. The AW lignites had greater specific heats than the raw lignites over the entire temperature range (lignites A and C) or at 50−150 °C (lignite B). Assuming that the heat of thermochemical reactions was negligible, greater specific heat

was interpreted as a higher mobility of macromolecules. This was reasonably explained by the removal of AAEM species, and it also supported the enhanced plasticization of the lignite during briquetting by AW. It was found for lignites A, B, and D that both the raw and AW lignites had small peaks of the specific heat at 80−90 °C, which were attributed to endothermic events. Although not evidenced, such peaks would be due to so-called enthalpy relaxation, which was associated with dissociation and/or rearrangement of noncovalent bonds such as hydrogen bonds. The raw and AW lignites were reheated after the first heating from 50 to 200 °C and the subsequent cooling to 50 °C. In the second heating, though not shown in Figure 7, endothermic peaks were reproduced for the all raw and AW lignites. It was clear that desorption of water was not involved in the endothermic peaks. Effects of HT200 on the specific heat were different from those of AW and also had a variety. The specific heat of the lignite C was increased by HT200 in a manner similar to that by AW. Physical structural modification of the lignite C by HT200 thus enhanced the mobility of macromolecules but by a mechanism different from AW. Changes in the specific heat of lignites A and B by HT200 were not significant, and no direct evidence of the enhanced mobility of macromolecules by the treatment was obtained. It was, however, clear that the treatment removed endothermic peaks at 80−90 °C which appeared upon heating of the raw lignites. The loss of the endothermic peak probably due to enthalpy relaxation was a proof of physical modification of macromolecular structure of lignites A and B. HT200 of lignite D made its specific heat smaller, while the endothermic peak at 80−90 °C observed for the raw lignite was lost. The smaller specific heat was interpreted as that HT200 lignite D had less molecular mobility than the original lignite D. Changes in the specific heats of lignites A, B, and D by HT200 were seemingly inconsistent with more extensive plasticization of HT200 lignites than the raw lignites, which was supported by higher TSs of the briquettes. Such apparent inconsistency could be explained considering the effect of mechanical pressure on the molecular mobility of the lignites, 6614

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extensive plasticization and deformation/coalescence of particles under mechanical pressure were necessary for preparing a briquette and then coke with higher TS.

which has not been experimentally proven. HT200 lignites A, B, and D as well as C had greater molecular mobilities under the mechanical pressure of 128 MPa than did the corresponding raw lignites but similar or less when no mechanical pressure was applied as in the case of the DSC analysis. It was believed that the macromolecular structure was physically modified by HT200 through water-induced relaxation. The newly formed macromolecular structure was stable with less mobility at temperatures up to 200 °C unless mechanical pressure was applied. Previous researchers36−41 studied changes in the physical structure of coal on a molecular or macromolecular scale by its treatment in organic solvents such as chlorobenzene under conditions that minimized extraction. The solvent treatment caused conformational changes of macromolecules in the swelling process and finally formed a physical structure more stable than the original one in the solvent removal process. Such irreversible and physical structural change was evidenced by changes in the specific heat,38 solvent swelling ratio,37 pyridine extractability,36 and thermoplasticity.41 According to those facts reported previously, it was suggested that the decrease in the specific heat of the lignite brought about by HT200 was mainly due to formation of physically more stable macromolecular structures of the lignites. However, such stabilized macromolecular structure would undergo plasticization even more extensively than did the original structure at 200 °C and under mechanical pressure as high as 128 MPa, resulting in more significant particles’ deformation and coalescence. It was also probable that the water-induced stabilization of the macromolecular structure was associated with a decrease in the density at its scale. Macromolecules of the lignite, so as those in coals, would be in a volume-minimum state rather than an energy-minimum state.42 The physical rearrangement of the macromolecules could thus cause increase in so-called free volume in macromolecular solid, as shown in Figure 7, simultaneously with a decrease in the free energy. Based on the above discussion, a scheme of improvement of TS of coke by HT200 is presented in Figure 8. Some of the effects of HT200 positive on TS of coke involve speculations, but those are consistent with the experimental results.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was carried out as a part of a research project, “Scientific Platform of Innovative Technologies for Coupgrading 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 and technical supports.

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REFERENCES

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4. CONCLUSIONS Cokes were prepared from four Indonesian lignites by a sequence of pretreatment (HT and/or AW), hot briquetting at 200 °C and 128 MPa, and carbonization with peak temperature of 900 °C. The hot briquetting and carbonization, even without pretreatment, produced cokes with TS of 7−22 MPa. AW removed AAEM species from the lignites, promoted their plasticization during the briquetting, enhanced the tar formation during the carbonization, and thereby increased TS of the resulting cokes to 18−24 MPa. The hydrothermal treatment at 200 °C (i.e., HT200) hardly removed AAEM species from the lignites, changed their chemical structure to a negligible degree, but caused physical change in the macromolecular structure, which enhanced plasticization of the lignites during the briquetting and resulted in the formation of cokes with TS of 13−36 MPa. Combination of HT200 and AW further increased TS of the cokes to 27−40 MPa. For each of the Indonesian lignites, TS of coke was linearly related to its bulk density. In other words, preparation of a coke with lower porosity was essential for production of a coke having higher TS. There were in fact linear relationships between TS of briquette and that of coke. It was clear that more 6615

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