Supercritical gas extracts from low-quality coals: on the search of new precursors for carbon materials

Fuel Processing Technology 86 (2004) 205 – 222 www.elsevier.com/locate/fuproc

Supercritical gas extracts from low-quality coals: on the search of new precursors for carbon materials Roberto Garcı´a *,1, Ana Arenillas 1, Fernando Rubiera, Sabino R. Moinelo Instituto Nacional del Carbo´n (INCAR), CSIC, Apartado 73, 33080, Oviedo, Spain Received 24 February 2004; received in revised form 2 March 2004; accepted 2 March 2004

Abstract This paper studies the chemical composition of several supercritical gas (SCG) extracts and its influence on the thermal behaviour under carbonisation conditions. The extracts were obtained from a Spanish lignite (Mequinenza), a low-quality coal from the point of view of energy applications. The lignite was treated with toluene, ethanol (EtOH) and tetrahydrofuran (THF) as solvents under different supercritical temperature and pressure conditions. The extracts display high aliphatic nature and enhanced concentrations of oxygen functional groups, aided by the contribution of hydrogenation and oxygen incorporation reactions occurring in the SCG extraction with EtOH and THF. Thiophenic compounds are also present in great concentrations derived from the exceptionally high organic sulphur content of the parent coal. The carbonisation of the extracts renders anisotropic material with fine mosaic texture, as a consequence of the significant thermal reactivity inferred by the aliphatic and oxygenated groups. The size of the mosaic increases with the temperature of the SCG extraction and varies with the supercritical solvent in the order: toluene < EtOH < THF. D 2004 Elsevier B.V. All rights reserved. Keywords: Supercritical gas extracts; Carbonisation; Thermal reactivity; Mesophase

1. Introduction Coal liquid derivatives are commonly used as precursors of carbon materials of different properties and for different applications [1– 7]. Coal tar pitches are the most common precursors in numerous cases, but availability is decreasing day by day as a * Corresponding author. Tel.: +44-115-951-4198; fax: +44-115-951-4115. E-mail address: [email protected] (R. Garcı´a). 1 Present address: School of Chemical, Environmental and Mining Engineering (SChEME), The University of Nottingham, University Park, Nottingham NG7 2RD, UK. 0378-3820/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2004.03.002

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consequence of the declining of coking industry, source of the coal tar feedstock. Nowadays, both the quality and the production of coal tar are decreasing at a constant rate, due to changes and improvements in steel production or environmental regulations on coke processing. Then, the search for new precursors is becoming mandatory, both to develop new carbon materials with different applications and to cover the demand of carbon materials derived from coal tar pitches. The use of coal liquids different from pitches as precursors of carbon materials remains as an incompletely studied area that will also open new possibilities to non-energetic coal uses. In the production of carbon materials, the carbonisation step plays a significant role. During carbonisation, the isotropic matrices of coal liquids are modified due to thermal decomposition and polymerisation reactions that may lead to the formation of the anisotropic mesophase [8– 13]. It appears in the form of spherules, in which formation, growth, coalescence and development are a result of the combined influence of the very complex chemical composition [14 –18] and physical properties of the coal liquid and the carbonisation conditions [8,19,20]. The main objective of this paper is to explore the possibilities of supercritical gas (SCG) extracts as precursors of carbon materials. The thermal behaviour on carbonisation of some extracts obtained by SCG extraction with different solvents is investigated. The formation of mesophase is monitored, together with its texture and morphology, and discussed on the basis of the chemical composition of the parent extracts. The special properties of supercritical fluids (high density, low viscosity) make them very appropriate as solvents for the extraction of low volatile solid compounds and allow them to easily penetrate in microporous structures. Hence, they are especially useful for coal extraction. The SCG extraction of high-sulphur coals with alcohols has been proposed as a route to obtain low-sulphur solid fuels for combustion [21 – 24]. The simultaneous thermal decomposition of coal and alcohols during SCG extraction has been found to promote hydrogen transfer, alkylation and esterification reactions [25 –27], which aid the extraction process and may have some bearing on sulphur removal. In this sense, SCG extraction might arise as a relevant practice for processing low-quality coals, such as the high-sulphur ones. The process would render a solid fuel with lower and acceptable sulphur content and an extract potentially suitable for further synthesis of carbon materials. This extract will display a high sulphur content, and evidences of a catalytic effect induced by the presence of sulphur when high-temperature treatments (1400 – 2000 jC) are carried out for obtaining the final carbon material have been previously reported [28]. This may lead to structural microdeformations [29], but any negative implications can be overcome if treatment is conducted until sufficiently high temperatures (2000 jC) [29,30].

2. Experimental section 2.1. Coal With the aim of investigating applications for low-quality coals, the coal selected in this study was the Spanish Mequinenza lignite (ML), which displays high organic sulphur

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content. Its analytical data are as follows: ash content, 21.6% dry basis; volatile matter (VM) content, 55.3% dry ash-free (d.a.f.); C content, 76.6% d.a.f.; H content, 5.0% d.a.f.; organic S content, 11.4% d.a.f.; N content, 0.9% d.a.f.; O content (calculated by difference), 6.1% d.a.f.; C/H atomic ratio, 1.28. Forms of sulphur (dry basis): pyritic, 0.9%; sulphatic, 0.1%; organic, 8.9%. 2.2. Supercritical gas (SCG) extraction The original coal was subjected to semicontinuous SCG extraction using toluene, ethanol (EtOH) and tetrahydrofuran (THF) in the experimental setup described elsewhere [27], under the conditions listed in Table 1. In a typical experiment, the 154 cm3 stainless steel reactor was loaded with 50 g of coal ground to less than 3 mm. After purging and pressure testing using argon, the solvent was pumped at 9 cm3 min
1 through the coal bed and the reactor was heated to the desired temperature using a fluidised bed sandbath. Extract and gases were collected during the whole experiment. After 180 min, the solvent flow was stopped and the solid residue or char removed from the reactor. The solvent was removed from the extract by rotary evaporation. Extract yields are also listed in Table 1. 2.3. Carbonisation of the extracts The SCG extracts ( c 0.25 g) were sealed in Pyrex tubes (8 cm height, 0.5 cm diameter). Inert atmosphere was achieved by making vacuum and filling with argon for three times before sealing. The loaded tubes were heated in a furnace at 5 jC min
1, up to a final temperature of 450 jC, and maintained at that temperature for 8 h. Then, the furnace with the tubes was left cooling overnight, and the tubes opened when room temperature was reached. 2.4. Thermogravimetric analysis (TGA) The SCG extracts were subjected to temperature-programmed pyrolysis tests in a Setaram TGA 92 thermogravimetric analyser. Approximately 10 mg of sample, an argon flow rate of 50 cm3 min
1 ( z 99.999% purity) and a linear heating rate of 15 jC min
1 to heat the sample from room temperature to 1000 jC were used. Furthermore, the SCG

Table 1 Operating conditions and extract yields in the SCG extractions Extract

Solvent

Temperature (jC)

Pressure (atm)

Extract yield (% d.a.f.)

ML1 ML2 ML3 ML4 ML5 ML6 ML7

Toluene Toluene EtOH EtOH EtOH THF THF

350 350 250 250 350 350 350

70 150 100 150 100 100 150

31.4 40.0 14.6 25.6 34.8 45.5 60.6

d.a.f.: dry and ash-free basis.

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extracts and the residues obtained from them in the thermogravimetric analysis under argon were subjected to temperature-programmed combustion tests under the same conditions as the pyrolytic experiments but using air instead of argon as the flowing gas. A Balzers MSC 200 quadrupole mass spectrometer linked to the thermobalance (TG/ MS) was employed to record the gas evolution profiles. The optimisation of the coupling system is described elsewhere [31]. 2.5. Optical microscopy (OM) The semicokes obtained in the carbonisation of the SCG extracts were characterised by OM in order to study the morphology of the anisotropic material developed. A polarised light microscope Zeiss Axioplan was used for this purpose, with an oil immersion 50  objective. Semicokes were embedded in an epoxy resin, left overnight and then cut and polished for the microscopic examination of particle cross-sections. 2.6. Fourier transform infrared (FT-IR) spectroscopy The extracts were also analysed by FT-IR in a Perkin-Elmer spectrometer, mod. 1750, provided with a Perkin-Elmer computer, mod. 7500. A drop of THF solution of each sample was placed on a KBr pellet, evaporating the solvent afterwards by heating at 70 jC under argon until constant weight. All the analyses were carried out in duplicate and the spectra were corrected for scattering using two baselines (4000 – 1800 and 1800 –450 cm
1). 2.7. Solution state proton nuclear magnetic resonance (1H NMR) spectrometry The extracts were analysed by 1H NMR using a Bruker AC-300 spectrometer in deuterated chloroform solutions, using the following conditions: frequency, 300 MHz; spectrum width, 3600 Hz; data points, 16 K; digital resolution, 0.44 Hz point; pulse width, 2.0 As (13.7); acquisition time, 2.294 s; no. of transients, 64. The cut-off points for the different types of hydrogen were as follows: Har + OH (aromatic and phenolic hydrogen), 9– 6 ppm; Hal (aliphatic hydrogen), 4.5 – 0.5 ppm; Ha,2 (hydrogen in methylene groups linking aromatic rings), 4.5 – 3.5 ppm; Ha (aliphatic hydrogen in a position to an aromatic ring or other functional groups), 3.5– 1.8 ppm; Hh (mainly aliphatic hydrogen in h position to an aromatic ring or other functional groups), 1.8 – 1.0 ppm; Hg (mainly aliphatic hydrogen in g position or further to an aromatic ring or other functional groups), 1.0– 0.5 ppm. The results are expressed as percentage of the total sample, calculated with the following equation: Hi ¼

H  HiNMR 100

ð1Þ

where: Hi: concentration of Hi in the sample, expressed as percentage of the total sample (i = ar + OH, al, a,2, a, h, g). H: concentration of total H in the sample (determined by elemental analysis).

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HiNMR: concentration of Hi in the sample, expressed as percentage of hydrogen (determined by 1H NMR). 2.8. High-performance liquid chromatography (HPLC) The SCG extracts were analysed by HPLC, following the procedure developed by Martı´n et al. [32 –34], in order to determine the distribution of different polyaromatic hydrocarbon (PAH) classes. Four classes of PAH can be distinguished: (i) cata-condensed compounds substituted with heteroatomic and/or alkyl and aryl groups (Cata1); (ii) cata-condensed compounds with naphthenic groups and/or substituted with alkyl and aryl groups (Cata2); (iii) unsubstituted cata-condensed compounds (Cata3); and (iv) peri-condensed compounds (Peri). The HPLC analyses were carried out using a Hewlett Packard HP1100 system consisting of two columns (PL gel, 300  7.5 mm, i.d.) packed with poly(styrene/ ˚ , respecdivinylbenzene) copolymer of two different nominal pore sizes (500 and 100 A tively) and connected in series. A diode-array detector operating at 254 nm was used. The mobile phase was dichloromethane/methanol (9/1 vol.) with a flow rate of 1 cm3 min
1. 2.9. Fractionation in oils and asphaltenes The proportions of oils and asphaltenes in the SCG extracts were determined by extraction with n-hexane, as follows: 2 g of extract sample and 70 cm3 of n-hexane were placed in a flask and reflux heated for 4 h. Then, the flask content was filtrated with a 5 Am pore size filter and washed with n-hexane. The asphaltenes (n-hexane insolubles) were dried at 50 jC under a slight argon flow. The filtrated solution was concentrated by rotary evaporation, and the solvent was completely removed from the oils (n-hexane solubles) by heating at 50 jC under a slight argon flow.

3. Results and discussion 3.1. Nature of the SCG extracts The significant aliphatic content of all the SCG extracts is indicated by FT-IR spectroscopy (Fig. 1) and 1H NMR results (Table 2). The FT-IR spectra of all the extracts display a band of methylene groups bending at 1465 cm
1 more intense than that of the aromatic CUC stretch at 1600 cm
1. The 6 – 8 times higher proportion of Hal over Har + OH, shown in Table 2, is in agreement with this result. Furthermore, the band at 1375 cm
1, due to the bending of methyl groups, displays a lower relative intensity than the 1465 cm
1, and the sum of the concentrations of Hh end Hg is markedly higher than the concentration of Ha. All these observations suggest the presence of high concentrations of aliphatic chains longer than one carbon and hydroaromatic groups, although the latter are less significant according to the Ha,2 contents. The composition evidenced by HPLC, shown in Table 3, is very similar in all the extracts, with a very abundant Cata1 fraction composed by cata-condensed polyaromatic compounds with all kind of substituents (alkylic, arylic and heteroatomic).

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Fig. 1. FT-IR spectra of Mequinenza lignite SCG extracts.

On the other hand, the C/H atomic ratio (Table 4) turns out to be influenced by the solvent used in the extraction, decreasing in the order: toluene>EtOH>THF. Beyond the dissolution capacity of the solvents, these results reflect the coal/solvent chemical interaction occurring under supercritical conditions, and more precisely the hydrogenation reactions taking place in the case of EtOH and THF, as previously reported [27,35]. These interactions also influence the extract yields of the SCG extraction, especially in the case of THF, which gives rise to significantly higher values under similar conditions (Table 1). Furthermore, in the case of EtOH extracts, it can be observed that C/H decreases with the increasing extraction temperature (ML3 and ML5, Table 4), due to the enhancement of hydrogenation and thermal cross-link breakage reactions [27,35], with a concomitant increase of the extract yield (Table 1). This is confirmed when the FT-IR spectra of ML3 and ML5 are compared (Fig. 1), as a significant increase of the intensity of the aliphatic

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Table 2 Contents (%) of hydrogen forms in the SCG extracts, calculated with Eq. (1) and using the values determined by 1 H NMR and the hydrogen content determined by elemental analysis Sample

Har + OH

Hal

Ha,2

Ha

Hh

Hg

ML1 ML2 ML3 ML4 ML5 ML6 ML7

0.87 0.81 0.87 0.97 0.97 0.94 1.06

6.64 6.49 6.19 5.93 6.76 8.22 7.40

0.11 0.18 0.20 0.30 0.27 0.83 0.32

2.02 2.04 1.40 1.86 2.38 3.44 2.54

3.47 3.35 3.25 3.00 3.33 3.13 3.59

1.04 0.92 1.34 0.77 0.78 0.82 0.95

Har + OH: aromatic and phenolic hydrogen; Hal: aliphatic hydrogen; Ha,2: hydrogen in methylene groups linking aromatic rings; Ha: aliphatic hydrogen in a position to an aromatic ring or other functional groups; Hh: mainly aliphatic hydrogen in h position to an aromatic ring or other functional groups; Hg: mainly aliphatic hydrogen in g position or further to an aromatic ring or other functional groups.

CUH band at 1465 cm
1 is observed in the high-temperature extract, ML5. The effect is also observed in the region of stretching of aliphatic CUH bonds, just below 3000 cm
1, the increase of the extraction temperature originates an increase of the intensity of the bands arising from methyl groups (2950 cm
1) relative to that due to methylene groups (2920 cm
1). The influence of pressure, however, is the opposite; the increasing pressure enhances the dissolution capacity of any of the supercritical solvents [36 – 38], which become able to extract higher proportions of higher-sized coal components, with higher aromatic condensation, thus increasing the extract yield (Table 1). This is also reflected in the proportions of oils and asphaltenes of the extracts (Table 5). For the same solvent, the asphaltene content increases with pressure, while the content of lower-sized oils increases with temperature. In both cases, the increase of the extract yield is observed (Table 1). In the area of the stretching of CMO groups, around 1700 cm
1 there is a dominant feature in all the FT-IR spectra centred at c 1710 cm
1 (Fig. 1), attributable to ketone groups. However, in the extracts obtained with EtOH (ML3, ML4 and ML5), a significant shoulder can be observed at c 1735 cm
1, arising from ester groups. In accordance with this, O/C atomic ratios reach the highest values in the extracts obtained with EtOH (Table Table 3 Distribution in classes of compounds (as determined by HPLC) of the SCG extracts, expressed as percentage of the whole extract Sample

Cata1 (%)

Cata2 (%)

Cata3 (%)

Peri (%)

ML1 ML2 ML3 ML4 ML5 ML6 ML7

90.52 92.14 90.90 91.05 92.57 93.61 93.53

4.11 4.53 5.12 5.18 4.11 3.47 2.94

1.08 0.99 1.07 1.14 0.85 0.78 0.76

4.29 2.34 2.91 2.63 2.48 2.15 2.78

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Table 4 Elemental analysis data of the SCG extracts, expressed as percentage of the whole extract Sample

C (%)

H (%)

S (%)

N (%)

O (%)a

C/H

S/C

O/C

ML1 ML2 ML3 ML4 ML5 ML6 ML7

73.90 73.34 69.02 68.43 71.13 72.65 71.15

7.51 7.30 7.06 6.90 7.73 9.16 8.46

8.16 7.88 10.40 10.07 8.43 7.34 7.22

0.92 0.32 0.67 0.78 1.02 0.80 0.87

9.51 11.16 12.85 13.82 11.69 10.05 12.30

0.82 0.84 0.81 0.83 0.77 0.66 0.70

0.041 0.040 0.057 0.055 0.044 0.038 0.038

0.097 0.114 0.140 0.151 0.123 0.104 0.130

a

Calculated by difference.

4), as a consequence of the incorporation of the solvent through esterification reactions [24,27,35]. The S/C atomic ratio is also higher in EtOH extracts due to the selective extraction of organic sulphur compounds [24,27,35]. 3.2. Carbonisation of the extracts Differences in the nature of the SCG extracts, influenced by the conditions of extraction (more remarkably, the solvent used), determine the yield of semicoke in the carbonisation experiments (Table 6). The lowest semicoke yields are observed in the case of extracts obtained with THF (ML6 and ML7). It has been mentioned above that among the supercritical solvents employed in this study, THF is the one that gives rise to extracted materials with lower C/H atomic ratio (Table 4), higher Cata1 content (Table 3) and higher content of aliphatic hydrogen, specially hydrogen atoms bonded to carbons adjacent to more than one aromatic ring (Ha,2, Table 2); from all these proofs, it is inferred a higher content of light components and/or a higher thermal reactivity, both leading to a more significant release of volatile matter during the carbonisation process. When observed under the polarised light optical microscope, all the semicokes obtained in the carbonisation of the SCG extracts display anisotropic material with fine mosaic structure, with a very homogeneous distribution. This is in accordance with the significant presence of aliphatic, hydroaromatic and heteroatomic structures and groups, revealed by the chemical data of Tables 2 –4 and Fig. 1, all of them foreseeing a high thermal

Table 5 Contents of asphaltenes and oils in the SCG extracts Extract

Asphaltenes (%)

Oils (%)

ML1 ML2 ML3 ML4 ML5 ML6 ML7

46.5 67.3 83.4 87.8 48.1 41.8 64.1

53.5 32.7 16.6 12.2 51.9 58.2 35.9

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Table 6 Semicoke and volatile matter yields in the carbonisation tests of the SCG extracts Sample

Semicoke yield (%)

Volatile matter yield (%)

ML1 ML2 ML3 ML4 ML5 ML6 ML7

69.93 77.48 59.79 71.55 73.55 50.08 54.60

30.07 22.52 40.21 28.45 26.45 49.92 45.40

reactivity. For example, HPLC fraction Cata1 has revealed in a previous study as the most thermally reactive under carbonisation conditions, leading to fast developments of mesophase [39]. Compared to pitches, this fraction only reaches similar levels of concentration in biomass pitches [40], while in petroleum pitches rarely exceeds 30%, and coal tar pitches display Cata1 concentrations well below 20% (except in the case of low-temperature coal tar pitches, with typical concentrations around 65%) [34,39,40]. As a consequence, the carbonisation of SCG extracts leads to a rapid formation of high molecular weight compounds with a three-dimensional, cross-linked structure that hinder stacking and solidify before mesophase growth and coalescence proceed to a significant extent [39,41]. On the other hand, it has been previously reported that the presence of sulphur-, nitrogen- and oxygen-containing heterocycles induce the formation of mesophase of small-sized optical texture [9,42,43]. The presence of such kind of heterocycles in the supercritical gas extracts, especially thiophenic compounds [24,27,35], undoubtedly contributes to the formation of fine-grained mosaics in the semicokes obtained in this study. However, some differences can be observed as a function of the supercritical solvent and the conditions used in the SCG extraction: – The size of the mosaic increases in the order: toluene (Fig. 2a) < EtOH (Fig. 2b) < THF (Fig. 2c). – The size of the mosaic increases with the temperature of the supercritical gas extraction: ML3 (Fig. 3a) c ML4 (Fig. 3b) < ML5 (Fig. 3c). Then, the texture size of the mesophase follows a trend: it increases with the decreasing C/H atomic ratio of the extracts and also with the increasing content of fraction Cata1 and the increasing proportion of aromatic and aliphatic hydrogen. Previous studies on coal liquids have reported correlation between hydrogen aromaticity and mesophase development, with higher aromatic hydrogen contents giving rise to higher texture size in the mesophase [44,45]. Hydrogen transfer reactions act as stabilisers of the radical reactive species formed, leading to polyaromatic precursors of mesophase [42]. The higher aromatic hydrogen contents of the THF supercritical extracts may be the explanation of the higher sized mosaics observed in their semicokes.

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Fig. 2. Optical micrographs of the semicokes of several extracts: (a) ML2 (obtained with toluene at 350 jC and 150 atm); (b) ML5 (obtained with EtOH at 350 jC and 100 atm); (c) ML7 (obtained with THF at 350 jC and 100 atm).

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Fig. 3. Optical micrographs of the semicokes of several extracts: (a) ML3 (obtained with EtOH at 250 jC and 100 atm); (b) ML4 (obtained with EtOH at 250 jC and 150 atm); (c) ML5 (obtained with EtOH at 350 jC and 100 atm).

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3.3. Thermogravimetric analysis The pyrolytic treatment of several SCG extracts of the Mequinenza lignite revealed differences in the global mass loss and in the temperature of maximum mass loss (Tmax). In accordance with the results discussed above, the supercritical solvent, which determines the chemical nature of the extracts, also governs the thermogravimetric results. Tmax values follow the order THF < EtOH < toluene (Table 7). As mentioned above, toluene extracts give rise to the lowest size of the mosaic texture observed in the carbonisation tests, and this could be related to a higher viscosity in the reaction media, preventing the coalescence and growth of the mesophase. In accordance with this, the highest value of Tmax observed in these extracts could be an indication of the presence of higher molecular size and thus, more thermally stable components. The corresponding DTG profiles (Fig. 4a) display a dominant feature centred at around 420 jC. At lower temperatures, another peak can be observed, starting at 150 jC and with a maximum at 250 jC. The small peak observed at 100 jC is attributable to moisture and rests of the supercritical solvent used. In the case of the toluene extract (ML2), the latter is more intense and develops up to higher temperatures, with a maximum at 150 jC, probably due to the presence of a more significant amount of remaining solvent. This may also be the explanation for the mass loss observed in the pyrolytic TG experiment of this extract, higher than those observed with the extracts obtained with THF and EtOH (Table 7). When the combustion profiles of the SCG extracts are considered, three peaks can be observed (Fig. 4b). The first two peaks resemble almost exactly the profile obtained in the pyrolysis tests: a low-temperature peak centred at 250 jC and a more significant peak, appearing at higher temperature, with a maximum at 420 jC. The third important mass loss observed in the combustion profiles also produces an intense peak centred at c 600 jC. It is deduced that the first significant peak (420 jC) represents the evolution of high volatile compounds (present originally in the samples or derived from thermal fragmentation reactions), released under both pyrolytic and oxidation conditions. The 600 jC peak, however, represents the evolution of compounds produced in the combustion of the pyrolysis residues. In fact, in the combustion profiles obtained for the residues of the pyrolysis tests, only this third peak is observed (Fig. 4c). Hence, it arises from the combustion of compounds that are resistant to thermal decomposition although temperature rises up to 1000 jC. In relation to the compounds evolved from the pyrolysis and combustion tests of several SCG extracts of Mequinenza lignite, and during the combustion tests of their pyrolysis residues, the evolution of oxygen (CO and CO2) and sulphur (SO2) compounds Table 7 Mass loss and Tmax as determined in the pyrolytic TGA tests of several SCG extracts of Mequinenza lignite Extract

Solvent

Temperature (jC)

Pressure (atm)

Mass loss (%)

Tmax (jC)

ML2 ML5 ML7

Toluene EtOH THF

350 350 350

150 100 150

84.81 76.69 78.61

436 428 419

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Fig. 4. DTG profiles of several SCG extracts: (a) pyrolysis; (b) combustion; (c) combustion of pyrolysis residues.

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has been studied. CO and CO2 profiles display roughly the same trends. Hence, for the sake of simplicity, only CO2 plots will be discussed here. Fig. 5 displays the evolution of CO2. The decomposition of oxygen functional groups promotes a maximum of CO2 evolution at c 420 jC observed under inert gas conditions (Fig. 5a), which indicates the significance of these decomposition reactions in the thermal behaviour of the extracts. This peak is more intense in the case of extracts ML5 and ML7 obtained with EtOH and THF, respectively, and containing a significant amount of oxygen groups incorporated during the SCG extraction process. The higher size of the mosaics observed in the semicokes of these extracts suggests that the high reactivity of these oxygen functional groups can contribute to the formation of mesophase, as already observed in a previous study [39], although the presence of H-donor groups must have also an influence. The decomposition of oxygen functional groups contributes to the main evolution of volatile matter observed under inert conditions (Fig. 4a). Obviously, the evolution of CO2 at 420 jC under combustion regime is only a shoulder of the main combustion CO2 peak centred at c 630 jC (Fig. 5b), corresponding to the burning of more condensed structures. In fact, this is the only CO2 evolution peak observed in the combustion of the pyrolysis residues (Fig. 5c). Fig. 6 displays the evolution profiles corresponding to SO2. Some evolution of SO2 under pyrolytic conditions is observed (Fig. 6a), especially in the case of the THF extract ML7, indicating some kind of interaction between sulphur compounds (mainly thiophenic) [24,35] and oxygen functional groups. However, the intensity of this evolution is low when compared to the evolution observed in the combustion test (Fig. 6b). Consequently, significant amounts of sulphur compounds have to be still present in the semicokes obtained at 450 jC in the present study (ML1 –ML7). This stability of sulphur structures could be an important feature for a subsequent utilisation of the SCG extracts. Two peaks of similar intensity are observed in the combustion tests of the three extracts analysed, centred at c 450 and c 650 jC, respectively. Comparing pyrolysis and combustion profiles (Fig. 6a and b), it seems that the presence of oxygen in the reacting gas enhances the evolution of SO2 at 450 jC. The almost negligible evolution of H2S observed during the pyrolytic TGA experiments rules out the presence of significant concentrations of the more thermally unstable thiols or thioethers, and leaves thiophenic groups as the most probable source of SO2 under combustion conditions. Differences in the condensation degree (thiophenes, benzothiophenes, dibenzothiophenes or further) and, then in thermal stability, account for the presence of two peaks of similar intensity. The first evolution peak is not observed in the combustion profiles of the pyrolysis residues (Fig. 6c), indicating that only the more thermally stable compounds remain after the pyrolysis TGA test. However, the latter have definitely undergone further stability-increasing condensation/polymerisation reactions in the pyrolysis test, giving rise to a broad SO2 evolution in the subsequent combustion. In any case, this second group of heterocyclic sulphur compounds, whose presence is deduced from the SO2 evolution in the combustion tests (Fig. 6b), is definitely present in the carbonisation semicokes of the SCG extracts obtained at 450 jC, contributing to the low extent of coalescence of the mesophase, leading to the fine mosaic texture [9,42,43], as mentioned above.

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Fig. 5. TG/MS profiles for CO2 of several SCG extracts: (a) pyrolysis; (b) combustion; (c) combustion of the pyrolysis residues.

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Fig. 6. TG/MS profiles for SO2 of several SCG extracts: (a) pyrolysis; (b) combustion; (c) combustion of the pyrolysis residues.

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4. Conclusions The SCG extracts display a high content of aliphatic and oxygen functional groups, leading to a high thermal reactivity. This is enhanced by hydrogenation and incorporation of oxygen groups occurring in the extraction with EtOH and THF. Accordingly, the most thermally reactive Cata1 fraction is present in very high concentrations. The high thermal reactivity produces anisotropic material with a texture of fine mosaic with a very homogeneous distribution under carbonisation. The size of the mosaic increases with temperature and varies with the supercritical solvent employed in the extraction following the order: toluene < EtOH < THF. DTG profiles under inert gas conditions reveal that the main mass loss occurs in an interval of temperatures centred at 420 jC. The evolution of CO2 is associated to the same mass loss event, indicating the key role played by the decomposition of oxygen functional groups in the development of mesophase. The high sulphur content of the studied SCG extracts is mainly distributed in thiophenic compounds of different aromatic condensation degree. These compounds, which are resistant to the carbonisation conditions employed, are transferred to the final semicokes and contribute to the fine mosaic anisotropic texture obtained.

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