Effects of Preheating and Oxidation on Two Bituminous Coals

948

Energy & Fuels 1996, 10, 948-957

Effects of Preheating and Oxidation on Two Bituminous Coals Assessed by Synchronous UV Fluorescence and FTIR Spectroscopy J. Kister and N. Pieri Universite´ d’Aix Marseille III, URA-CNRS 1409, 13397 Marseille, France

R. Alvarez, M. A. Dıéz,* and J. J. Pis Instituto Nacional del Carbo´ n (INCAR), C.S.I.C., Apartado 73, 33080-Oviedo, Spain Received August 7, 1995. Revised Manuscript Received May 8, 1996X

Modifications induced by the preheating process in the distribution of families of polyaromatic hydrocarbons (PAHs) in chloroform and pyridine extracts of two bituminous coals from Spain have been determined by synchronous UV fluorescence spectroscopy. Using PAH model compounds, three main synchronous fluorescence spectral regions can be distinguished according to the number of lineally condensed aromatic rings. The differences due to the different rank of the two parent coals (wet coals) are retained when they are subjected to oxidation and preheating prior to oxidation. Using chloroform and pyridine as extracting solvents, the lighter PAHs with two- or three-rings (mainly with one naphthalene skeleton) and the relatively larger and heavier PAHs with four- or more aromatic rings appear to be the compounds most affected by preheating of the highest rank coal. The differences between the parent and preheated coals in the composition of the extractable material are in good agreement with results from Fourier transform infrared structural studies, reflecting the relative higher aromaticity and degree of condensation of polyaromatic units, as well as the geochemical parameters of the entire organic matrix of these coals, such as the H/C and O/C atomic ratios. The oxidation effects of the wet and preheated coals have also been studied using the same methodology by synchronous fluorescence and FTIR spectroscopic techniques. When the preheated coals were oxidized a different behavior was observed between wet and preheated coals. This was due to structural changes caused by the preheating at about 200 °C. The fluorescence data suggest that the highest rank coal (Mieres) after preheating had a similar oxidation response to Mieres wet coal, whereas the preheatinginduced structural changes in Nalon coal provide fewer active sites to be oxidized.

Introduction Preheating of coal at about 200 °C in an inert atmosphere prior to carbonization in coke ovens is a procedure used in the production of metallurgical coke for the blast furnace. This technology was used by the coking industry in the 1970s, but the trend declined in the 1980s. Currently, however, the preheating of coal in combination with the dry cooling of the resulting coke is being incorporated into the operation of the “Jumbo Coking Reactor” (JCR). This is a European Eureka Research Project important for the future of the cokemaking technology.1,2 In general, most studies of preheating have focused on advantages in terms of technological improvements to coke quality and coke productivity, together with the widening of the range of coals suitable for coking compared to conventional wet gravity charging.3-7 Industrial preheating produces a Abstract published in Advance ACS Abstracts, June 15, 1996. (1) Nasham, G. Proceedings of the 1st International Cokemaking Congress, Essen, Germany, European Cokemaking Comm.; Verlag Gluckauf GmbH: 1987; Vol. 1, p A2. (2) Nasham, G. Cokemaking Int. 1990, 2, 19-29. (3) Eisenhut, W. Chemistry of Coal Utilization; Elliot, M. A., Ed.; John Wiley & Sons: New York, 1981; 2nd Suppl. Vol., Chapter 14, pp 847-917. (4) Beck, K. G. The Year-Book of the Coke Oven Managers’ Association (COMA), Mexborough, U.K., 1974; p 230. X

S0887-0624(95)00159-9 CCC: $12.00

decrease in the volatile content of the preheated coal compared to the original coal due to a devolatilization of some of the coal particles which are accompanied by coal particles with pore formation.8 However, relatively little attention has been given to determining the structural changes induced in coals by the industrial preheating process. Coal may be considered a three-dimensional crosslinked macromolecular structure in which relatively lower molecular weight compounds are trapped in the pores.9 These small molecules have been defined as the mobile component in coals, and they are extractable using organic solvents.10-12 Brown and Waters have shown that the mobile component extracted with CHCl3 (5) Alvarez, R.; Alvarez, E.; Sua´rez, C.; Meńdez de Ande´s, F.; Fernańdez, E.; Sirgado, M. Proceedings of the 1st International Cokemaking Congress, Essen, Germany, European Cokemaking Comm.; Verlag Gluckauf GmbH: 1987; Vol. 1, p E5. (6) Alvarez, R.; Dıéz, M. A.; Meneńdez, J. A.; Pis, J. J.; Sua´rez, C.; Sirgado M. Cokemaking Int. 1993, 5 (2), 36-40. (7) Dıéz, M. A.; Alvarez, R.; Sirgado, M.; Marsh, H. ISIJ Int. 1991, 3, 449-457. (8) Meneńdez, R.; Alvarez, R.; Alvarez, D.; Canga, C. S.; Dıéz, M. A.; Gonza´lez, A. I. Fuel 1992, 71, 1265-1270. (9) Dryden, I. G. C. In Chemistry of Coal Utilization; Lowry, H. H., Ed.; J. Wiley: New York, 1963; Suppl. Vol., Chapter 6, pp 232-295. (10) Jurkiewicz, A.; Marzec, A.; Pis´lewski, N. Fuel 1982, 61, 647650. (11) Marzec, A.; Jurkiewicz, A.; Pis´lewski, N. Fuel 1983, 62, 996998.

© 1996 American Chemical Society

Effects of Preheating and Oxidation on Bituminous Coals

Energy & Fuels, Vol. 10, No. 4, 1996 949

plays an important role in the development of coking ability of coals.13 Low-temperature treatment of coal in an inert atmosphere enhances the extractability of coals, and there is some evidence that a portion of the new soluble material found after heating was originally present in coals as a constituent but was somehow made accessible by a thermal process.13,14 Results from the characterization of the coal extracts using international coking coals with different rank15 confirm the irreversible changes which occur in the organic matrix16 and pore structure8 of coal during the industrial preheating process and, consequently, explain the different behaviors of the preheated coals during carbonization. Structural changes in the organic matter of coal induced by weathering or oxidation also affect its coking properties, its behavior in the coking process, and hence coke quality.17 Numerous studies have been carried out on weathering or artificial oxidation.18-20 Synchronous UV fluorescence (SF) spectroscopy may provide useful information concerning structural modifications in the extractable aromatic part of preheated and oxidized coals. The approach of using the SF spectroscopic technique, which consists of scanning both excitation and emission wavelengths simultaneously at a fixed wavelength interval, can enhance the selectivity of fluorescence multicomponent analysis. Signals of synchronous fluorescence of PAHs are obtained at a wavelength range where excitation and emission overlap,21-24 resulting in a much higher spectral resolution of the complex mixtures compared to fixed wavelength monitoring in conventional fluorescence.25 Thus, PAHs usually exhibit only a strong single or occasionally double peak, varying in wavelength according to the structure.21-24,26-32 This technique has been successfully applied in the determination of PAH families or certain PAHs in coal-derived liquids and coal extracts in organic solvents with different extractive ability.26,27,29,31,32

Table 1. Main Characteristics of Mieres and Nalon Coals (Wet and Preheated)

(12) Given, P. H.; Marzec A.; Barton W. A.; Lynch L. T.; Gerstein B. C. Fuel 1986, 65, 155-163. (13) Brown, H. R.; Waters, P. L. Fuel 1966, 45, 17-39, 41-59. (14) Hishioka, M.; Larsen, J. W. Energy Fuels 1988, 2, 351-355. (15) Kister, J.; Pieri, N.; Dıéz, M. A.; Alvarez, R.; Pis, J. J. In Coal Science and Technology 24; Coal Science Vol. I; Pajares, J. A., Tascoń, J. M. D., Eds.; Elsevier: Amsterdam, 1995; pp 381-384. (16) Kister, J.; Ruau, O.; Landais, P.; Alvarez, R.; Dıéz, M. A.; Pis, J. J. Fuel Process. Technol. 1993, 36, 313-318. (17) Pis, J. J.; Cagigas, A.; Simon, P.; Lorenzana, J. J. Fuel Process. Technol. 1988, 20, 307-316. (18) Fredericks, M. P.; Warbrooke, P.; Wilson, M. A. Org. Geochem. 1984, 5, 89-97. (19) Nelson, C. R., Ed. Chemistry of Coal Weathering; Coal Science and Technology 14; Elsevier: Amsterdam, 1989. (20) Davidson, R. E. Natural oxidation of coal. IEA Coal Research IEACR/29; London, U.K., 1990. (21) Lloyd, J. B. F.; Evett, I. W. Anal. Chem. 1977, 49 (12), 17101715. (22) Vo-Dinh; T. Anal. Chem. 1978, 50, 396-401. (23) Lloyd, J. B. F. Analyst 1980, 105 (17), 97-108. (24) Baudot, Ph.; Viriot, M. L.; Andre´, J. C.; Jezequel, J. Y.; Lafontaine, M. Analusis 1991, 19, 85-97. (25) McKay, J. F.; Latham, D. R. Anal. Chem. 1972, 44, 2132-2136. (26) Katoh, T.; Yokoyama, S.; Sanada, Y. Fuel 1980, 59, 845-850. (27) Vo-Dinh, T.; Martıńez, P. R. Anal. Chim. Acta 1981, 125, 1319. (28) Vo-Dinh, T.; Gammage, R. B.; Martıńez, P. R. Anal. Chem. 1981, 53, 253-258. (29) von der Dick, H.; Kalkreuth, W. Fuel 1984, 63, 1636-1640. (30) Kerkhoff, M. J.; Files, L. A.; Winefordner, J. D. Anal. Chem. 1985, 57, 1673-1676. (31) Mille, G.; Guiliano, M.; Kister, J.; Org. Geochem. 1987, 13, 947952. (32) Benkhedda, Z.; Landais, P.; Kister, J.; Dereppe, J.-M.; Monthioux, M. Energy Fuels 1992, 6, 166-172.

Mieres coal ash (wt % db) volatile matter (wt % db) sulfur (wt % db) swelling index Arnu dilatometer Tr (°C) a + b (%) Gieseler plastometer Tr (°C) maximum fluidity (log ddpm) mean reflectance of vitrinite (R h o, %) standard deviation of R ho

Nalon coal

wet

preheated

wet

preheated

7.5 31.4 0.98 81/4

6.6 32.1 0.92 8

6.3 35.9 1.28 73/4

5.8 35.8 1.12 7 3 /4

347 231

352 172

341 135

344 101

384 4.00

385 3.37

386 4.03

393 3.43

0.96

0.96

0.84

0.84

0.14

0.14

0.07

0.07

In the present study, the distribution of different PAH classes assessed by SF spectroscopy provides information on the preheating-induced changes in composition that occur in the mobile component of coking coals. The results are interpreted by emphasizing differences between the chloroform (CHCl3) and pyridine (Py) extracts of the initial wet coals and the major transformations that occur during the preheating process. In addition, Fourier transform infrared (FTIR) spectroscopy provides complementary information on the structural changes in the organic matrix of coals after preheating. Attention is given to the effects of oxidation on the organic matter of wet and preheated coals. The correlation of H/C and O/C atomic ratios is also considered using a van Krevelen diagram, and the different behaviors of wet and preheated coal upon oxidation can be established. Experimental Section Two bituminous coals (W) (Mieres and Nalon) from Spanish mines were used for preheating (P), oxidation (Wox), and oxidation after preheating (Pox). Because grinding of coal creates fresh surfaces, the “natural” fraction < 0.15 mm in size (obtained by screening but not grinding), was selected in this study. Table 1 summarizes the main characteristics of the coals used. Industrial Coal Preheating. The wet coals (W) were preheated at about 200 °C in a 2 t/h Precarbon pilot plant. The Precarbon process includes a two-stage flash heater for drying and preheating the wet coal (W) under an inert atmosphere.3,4 Further details of the Precarbon pilot plant at INCAR are described elsewhere.5-7 Coal sampling was carried out (a) before charging the preheating pilot plant with the wet coals (W) and (b) before charging the coke oven within the closed conveyor with the preheated coals (P). For collection of a gross coal sample, the number, the spacing, and the weight of individual increments were assessed according to ISO standard procedures,33 which are quite similar to ASTM procedures. The gross sample obtained was on the order of 24 kg and then divided by using the riffles to give a representative sample for analysis of about 1 kg. Coal Oxidation. Three-gram samples of wet and preheated coals (fraction < 0.15 mm) were oxidized in a laboratory oven with forced circulation of air at 140 °C for 2 h. After oxidation, coal samples (Wox and Pox) were stored under argon to prevent additional oxidation. The oxidation temperature (33) International Organization for Standardization (ISO), 1975, ISO 1988-1975. Hard Coal-Sampling. (ASTM D2234-89, Standard Test Methods for Collection of a Gross Sample of Coal, and ASTM D201386, Standard Test Method for Preparing Coal Samples for Analysis).

950

Energy & Fuels, Vol. 10, No. 4, 1996 Table 2. Assignments of FTIR Bandsa frequency

(cm-1)

3100-2980 2980-2760 2950 2920 2855 1440 1375 900-710

major assignment ν(CH)ar ν(CH)al νas(CH3) νas(CH3, CH2) νs(CH2) δ(CH3, CH2) δ(CH3) γ(CH)ar

a

Subscripts: ar, aromatic; al, aliphatic; ν, stretching vibration; δ, deformation vibration in plane; γ, deformation vibration out of plane; as, asymmetric; s, symmetric.

was chosen in order to minimize the production of hydrocarbons in the oven.34 Fourier Transform Infrared (FTIR) Spectroscopy. All spectra were recorded on a Nicolet 20 SXB spectrometer using KBr standard pellets (1:150 coal to KBr ratio) by co-adding 100 interferograms. Each spectrum was normalized to 1 mg of sample. Mineral matter interferences were eliminated by subtracting the corresponding spectrum of the low-temperature ash (LTA). The main assignments of the FTIR bands used in this study are given in Table 2. The integration method and a detailed band assignment of the spectra of coals are described elsewhere.35 Selected indices derived from FTIR data using the integrated area (Area) or the maximum intensity (H) of different spectral bands were used. Coal Extraction. Two extraction procedures were used. All coal samples (W, P, Wox, and Pox) were ultrasonically extracted with chloroform, CHCl3 (0.1 g of solute/30 mL of solvent), for 45 min. Chloroform extraction yields were less than 2 wt %. Besides extraction with CHCl3, coal samples (W and P) were also ultrasonically extracted with pyridine, Py (5 mg of solute/80 mL of solvent), for 45 min. Pyridine extraction yields were higher than CHCl3 extraction yields but were less than 10 wt %. The exposure of coal to pyridine results in substantial swelling of the solid phase which allows heavier molecules to be removed following the rupture of labile bonds between aromatic units. This gives rise to extractable polyaromatic compounds that are not extracted with CHCl3, explaining the higher extract yield with pyridine. Extraction using pyridine was only performed on the wet and preheated coals. Synchronous UV Fluorescence (SF) Spectroscopy. After filtration and solvent evaporation, the extracted material was dissolved in tetrahydrofuran (THF is a suitable solvent for avoiding quenching effects) for synchronous UV fluorescence (SF) analysis using a concentration of 1/100 (v/v) for pyridine extracts and 5 ppm for CHCl3 extracts. Fluorescence measurements were carried out using a Perkin-Elmer LS50 luminescence spectrometer. The excitation and emission wavelengths were scanned simultaneously at a fixed wavelength interval (∆λ) of 23 nm. Emission and excitation slits were set at 5 nm, and the scanning speed was kept constant (200 nm/s). Optimization of the instrumental conditions for coal extracts has been reported in previous studies.31,32,36 The high dilution used minimizes quenching effects and ensures a high dependence between the structure of a given PAH family and the spectral profile. The assignments of the different fluorescence peaks on the spectra of the CHCl3 and pyridine extracts are given in Table 3. For quantification, the values of the area (for CHCl3 extracts) and the height (for (34) Michels, R.; Landais, P.; Gerard, L.; Kister, J. C. R. Acad. Sci. Paris 1993, 316, Se´rie II, 1375-1381. (35) Guiliano, M.; Mille, G.; Doumenq, P.; Kister, J.; Muller, J. F. In Advanced Methodologies in Coal Characterization; Coal Science and Technology 19; Charcosset, H., Ed.; Elsevier: Amsterdam, 1990; pp 399-417. (36) Mille, G.; Kister, J.; Doumenq, P.; Aune J. P. In Advanced Methodologies in Coal Characterization; Coal Science and Technology 15; Charcosset, H., Ed.; Elsevier: Amsterdam, 1990; Chapter 13, pp 235-251.

Kister et al. Table 3. Assignments of the Different Fluorescence Peaks peak

spectral range (nm)

main assignmenta

A1

300-370

A2

370-460

A3

460-580

two or three lineally condensed aromatic rings (PAHs with one naphthalene skeleton) three or four lineally condensed aromatic rings (PAHs with one anthracene skeleton and benzofluoranthene skeleton) four or more lineally condensed aromatic rings (PAHs with one tetracene skeleton and benzofluoranthene skeleton)

a Assignments of the different spectral regions include PAHs substituted by alkyl or naphthenic groups and heterocycle compounds with the same number of aligned aromatic rings.

pyridine extracts) of these peaks were used as indicators of the relative evolution of the molecular size and topology of the aromatic units.

Results and Discussion General Considerations of Assignments of SF Spectral Regions. The SF signals of certain PAHs that belong to different families are summarized in Figure 1. The data presented for a significant number of the model compounds were obtained under the same experimental conditions (∆λ ) 23 nm), and the remaining data collected from ref 24 correspond to SF signals at ∆λ of 10 nm. The choice of these compounds was made to cover a wide range of the spectra and to provide representative PAHs of the following families: alternant PAHs, catacondensed (linear and nonlinear), and pericondensed (Figure 1A), benzo derivatives of acenaphthene and fluorene and nonalternant pericondensed compounds (Figure 1B), and aromatic compounds containing N, S, and O (Figure 1C). According to the theory of molecular fluorescence, the PAHs with a different number of rings have different fluorescing properties. As a result the PAHs with more lineally condensed aromatic rings in the system fluoresce at longer wavelengths than those with fewer rings, i.e. naphthalene (compound 2 in Figure 1A), 321 nm; anthracene (7), 378 nm; and tetracene (16), 473 and 500 nm. In addition, other general considerations can be drawn from the data in Figure 1 and those published by other authors: (a) There is a significant bathochromic effect (shift to a longer wavelength) in the SF signal with increasing linear annellation, i.e. isomeric PAHs: chrysene (4), 341 nm; benzo[a]anthracene (8), 387 and 407 nm; and tetracene (16), 473 and 500 nm. (b) Side chains attached to aromatic rings and naphthenic rings will shift the SF signal to longer wavelengths. In general, the effect of alkyl substituents can be expected to decrease with the degree of condensation in the system and with the nature and degree of the substitution. Thus, all the alkylated PAHs give an SF signal in the region of the parent hydrocarbon but differ by a few nanometers, i.e. 1-methyl- and 2-methylnaphthalene (327 and 328 nm, respectively); 2,3,6-trimethylnaphthalene (334 nm) compared to 321 nm for naphthalene24 and 2-methyl- and 9-methylanthracene (386 and 395 nm, respectively) vs 382 nm for anthracene.24 However, there are certain substituted PAHs which display a significant bathochromic effect. This is the


Figure 1. SF data for standard PAHs. (A) Benzene/toluene (1); naphthalene (2); 1-phenylnaphthalene (3); chrysene (4); phenanthrene (5); pyrene (6); anthracene (7); benzo[a]anthracene (8); benzo[e]pyrene (9); dibenzo[a,h]anthracene (10); benzo[a]pyrene (11); benzo[ghi]perylene (12); anthanthrene (13); dibenzo[a,e]pyrene (14); perylene (15); tetracene (16). (B) Fluorene (17); acenaphthene (18); 1,2-benzofluorene (19); 2,3benzofluorene (20); 3-methylcholanthrene (21); benzo[b]fluoranthene (22); benzo[k]fluoranthene (23); fluoranthene (24); indeno[1,2,3-cd]pyrene (25). (C) Benzothiophene (26); dibenzothiophene (27); benzo[b]naphto[2,3-d]thiophene (28); phenanthro[4,3-d]thiophene (29); indole (30); acridine (31); 4-hidroxibenzoic acid (32); 1-naphthylacetic acid (33); 1-naphthol (34); 2-naphthol (35); 2-naphthalenethiol (36); dibenzofuran (37). Superscripts a-c: ∆λ ) 23 nm. Data taken from refs 32, 31, and 24, respectively. Superscript d: ∆λ ) 10 nm. Data taken from ref 24. Superscript e: experimental data at ∆λ ) 23 nm. Superscript s: shoulder.

case of 9,10-dimethylanthracene (414 nm) compared to 382 nm for the parent hydrocarbon, anthracene.24 A


bathochromic effect is observed for tetraline (310 nm) compared to benzene (270 nm). The incorporation of a second saturated six-membered ring produces a shift to longer wavelength of about 5 nm.26 In general, a naphthenic ring produces a bathochromic shift on the order of 10-30 nm. (c) Aromatic heterocycles containing N, S, or O produce a bathochromic effect compared to the corresponding PAH. This is the case of dibenzothiophene (27), the SF signal appearing on a longer wavelength of 35 nm than fluorene; dibenzofuran (compound 37: 304 nm) vs 299 nm for fluorene (17); benzo[b]naphto[2,3-d]thiophene (compound 28: 369 nm) vs 354 nm for 2,3-benzofluorene and acridine (compound 31: 415 nm) vs 378 nm for anthracene. It has been also established that the position of the nitrogen atom does not appear to have a significant effect on the fluorescence spectra of large nitrogen heterocycles.37 Nonlinear compounds, hydroaromatics, and those with alkyl substituents might be expected to follow, to a certain extent, the above basic rules. Because of the simplicity of the SF signal of each PAH, which shows generally one or a limited number of peaks within a definite spectral range, and taking into account the relation between the fluorescence signal and the structure, three main SF regions can be defined. The first spectral region covering the 300-370 nm wavelength range can be assigned to alternant PAHs which have in common either a naphthalene skeleton or the incorporation through a peripherical cisoid C4 arrangement of a phenanthrene structure (chrysene, compound 4). All of them are characterized by two aligned benzenic rings. The assignment of this region shows similarity with that reported by Katoh et al.26 using model compounds. The authors reported that the spectral region 270-310 nm corresponds to aromatic hydrocarbons that have only one aromatic ring and the next spectral region, 310-340 nm, belongs to those that have two condensed aromatic rings (one naphthalene nucleus).26 This assignment was successfully applied to the characterization of HPLC fractions of coal-derived liquids. Despite the wide type of PAHs (benzo derivatives of anthracene, pyrene and perylene) in the following spectral region (370-460 nm), they have a common characteristic. All PAHs in this region have at least one anthracene partial structure in the system. This means that these compounds always have three aligned benzenic rings, regardless of the presence of other nonlineally attached aromatic rings, i.e. catacondensed and pericondensed PAHs with four, five, and six rings in the aromatic cluster. Taking into account the alkyl or naphthenic substituted hydrocarbons, it can be assumed that most of these types of PAHs give an SF signal in the region of the unsubstituted PAH. Finally, the region beyond 460 nm, for which there are insufficient data, correspond to benzo derivatives of tetracene (four lineally condensed aromatic rings). It is probable that this region is also related to PAHs substituted by alkyl or naphthenic groups which contain fewer than four condensed rings. However, the degree of substitution in this case is high. In addition, substituted PAHs can be expected to produce a peak in this third region, (37) Kershaw, J. R. Fuel 1983, 62, 1430-1435.

952

Energy & Fuels, Vol. 10, No. 4, 1996

Figure 2. SF spectra of Mieres coal, wet (W): (a) CHCl3 extract and (b) pyridine extract.

the unsubstituted PAH showing an SF signal on the interface of the two regions (370-460 and >460 nm). As regards PAHs containing a five-member ring in the system, two different behaviors can be observed. The first corresponds to benzo derivatives of acenaphthene and fluorene (having one five-membered ring with one or two methylene groups) which give an SF signal corresponding the remaining benzenic rings, i.e. 1,2- and 2,3-benzofluorenes (compounds 19 and 20 in Figure 1B) give their SF signals in the range of naphthalene derivatives (352 and 354 nm, respectively, vs 321 nm). The second is related to nonalternant PAHs such as fluoranthene benzo derivatives in which the fivemember ring without methylene groups can be considered as a benzenic ring, i.e. 472 nm for indeno[1,2,3cd]pyrene. The above classification does not exclude overlapping between the above type of PAHs and those with a certain degree of substitution and with a lower number of condensed benzenic rings in a linear arrangement. All of the above observations are summarized in Table 3 in which the assignment for the two first peaks does not exclude overlapping between PAHs that contain n (2 and 3) and n + 1 linear condensed aromatic rings. The limits quoted for the wavelength ranges in Table 3 can vary in about 5 nm. SF Spectra of Coal Extracts. Figure 2 shows an example of the SF spectra recorded at ∆λ of 23 nm, corresponding to CHCl3 and Py extracts of Mieres W coal. All the SF spectra of the CHCl3 and Py extracts diluted in THF are very similar qualitatively, and differences appear more in relation with the area, intensity, and position of maximum intensity of the spectral peaks. Both CHCl3 and Py extracts are characterized by three different peaks/regions, which are assigned to different families of PAHs according to the number of lineally condensed aromatic rings (Table 3). Due to the similarity of the spectra and inherent limitations of the technique, the results presented in Table 4 can only be considered as an estimation of the relative abundance of each PAH family rather than as accurate quantitative data. The repeatability, as relative standard deviation (RSD), for the abundance of the different PAH families was found to be quite good.

Kister et al.

Thus, the percentages of A1 and A3 deviated by 4.3 and 2.7%, respectively, while the percentage of A2 had a deviation of less than 1% (for five different samples of CHCl3 extracts of Mieres W coal). In all cases, the spectra comprise a major peak corresponding to PAHs with three- and four-condensed rings (one anthracene skeleton), which are 56-65% for the CHCl3 extracts (Table 4) and 45-48% for the Py extracts (Table 4). The relative distribution of PAH families appears to be sensitive to the extracting solvent used, the coal rank, and the preheating and oxidation processes. Table 4 also contains the values of the fluorescence indices based on the ratio between the area or the maximum intensity of the spectral peaks (A1, A2, and A3) and the standard deviation for each one. The fluorescence indices defined clearly indicate the compositional differences toward lighter or heavier PAHs in the extracts due to the different rank, the preheating process, and the oxidation reaction. Comparison between Mieres and Nalon Coals. Comparison of the distribution of PAHs in the CHCl3 extracts, according to the number of lineally condensed aromatic rings, indicates that the Mieres coal has a higher proportion of PAHs (more than three aligned aromatic rings) than the Nalon coal (Table 4). The distribution of such PAHs is consistent with the higher rank of the Mieres coal and agrees with published results for a set of six coals ranging from lignite to semianthracite (samples from the GRECO-CNRS bank).38 A good correlation was found between fluorescence indices and rank parameters of selected coals (mean reflectance of vitrinite, H/C atomic ratio, and volatile matter content). The maximum intensity of the fluorescence peaks that correspond to PAHs with two- and three-condensed rings (λ1 between 345 and 360 nm) and three- and fourcondensed rings (λ2 between 370 and 460 nm) is located at longer wavelengths (bathochromic effect) for Mieres W coal (λ1 ) 359 and λ2 ) 402 nm) than for Nalon W coal (λ1 ) 346 and λ2 ) 395 nm) (Table 5). The occurrence of maximum intensity at longer wavelengths, which is consistent with higher rank, suggests an increase in the average molecular weight of the PAH families. The same trend was found for the pyridine extracts. This fact was also found in fluorescence emission spectra of coal-tar pitch fractions by Zander and Haenel.39 These authors reported a good linear relation between the number-average molecular weight as determined by vapor pressure osmometry and the maximum wavelength of the broad fluorescence band.39 For both measurements, differences of 2 nm indicate compositional changes in such PAH families. However, the maximum intensity of the third peak (λ3 between 460 and 580 nm) was not sufficiently defined for an accurate measurement to be made. To assess more easily the differences which exist between the original and preheated coal, the Py extracts were also analyzed by SF spectroscopy. Although differences in the distribution of PAHs are smaller, pyridine extracts show a trend identical to that found for the CHCl3 extracts. The Py extract of Mieres W coal is composed of a lower proportion of light PAHs (A1) (38) Kister, J.; Doumenq, P.; Davin, E.; Mille, G. C. R. Acad. Sci. Paris 1992, 315, Se´rie II, 149-152. (39) Zander, M.; Haenel, M. W. Fuel 1990, 69, 1206-1207.



Table 4. Distribution of the Different PAHs Families and Fluorescence Indices for Chloroform and Pyridine Extracts percentage of PAH familya A1

A2

fluorescence indicesa A3

I1 or R1

I2 or R2

I3 or R3

I4 or R4

Chloroform Extracts Mieres coal wet (W) preheated (P) wet oxidized (Wox) preheated oxidized (Pox) Nalon coal wet (W) preheated (P) wet oxidized (Wox) preheated oxidized (Pox)

18.0 20.4 20.1 21.7

63.0 65.2 62.8 64.9

19.0 14.4 17.1 13.4

3.487 3.203 3.127 3.066

0.301 0.220 0.268 0.187

1.050 0.706 0.837 0.572

0.234 0.168 0.203 0.141

32.0 31.3 32.1 28.9

58.1 58.7 56.8 60.9

9.9 10.0 11.1 10.2

1.816 1.873 1.773 2.108

0.169 0.170 0.196 0.168

0.308 0.318 0.347 0.355

0.109 0.111 0.125 0.114

Pyridine Extracts Mieres coal wet (W) preheated (P) Nalon coal wet (W) preheated (P)

22.0 9.2

47.9 45.4

30.1 45.4

2.181 4.923

0.629 1.000

1.372 4.923

0.431 0.831

24.1 25.3

48.5 45.9

27.4 28.8

2.014 1.810

0.564 0.627

1.137 1.135

0.377 0.404

a Fluorescence indices calculated from the area of peaks measured directly from the SF spectra for chloroform extracts (I ) and from n the height of peaks for pyridine extracts (Rn). I1 and R1 ) A2/A1; I2 and R2 ) A3/A2; I3 and R3 ) A3/A1; I4 and R4 ) A3/(A1 + A2); where A1, A2, and A3 correspond to the area or height of peak assigned to PAHs with two or three condensed aromatic rings, three or four condensed aromatic rings, and four or more condensed aromatic rings, respectively. The standard deviation for each index was 0.156 for I1, 0.008 for I2 and I3, and 0.075 for I4.

Table 5. Position of the Maximum Intensity of Fluorescence Peaks λ1 max

λ2 max

Mieres Coal wet (W) preheated (P) wet oxidized (Wox) preheated oxidized (Pox)

359 353 356 356

402 396 400 396

Nalon Coal wet (W) preheated (P) wet oxidized (Wox) preheated oxidized (Pox)

346 345 345 349

395 393 395 395

together with a higher proportion of the larger PAHs with four or more rings, highly substituted PAHs, or hydroaromatic compounds with fluorescent properties in the region A3 compared to Nalon P coal (Table 4). The proportion of PAHs with three and four lineally condensed rings (A2) is quite similar for both coals. Moreover, important differences can be deduced when the distributions of PAHs of the CHCl3 and Py extracts from the same coal are compared (Figure 3). An increase in the proportion of PAHs with four or more condensed aromatic rings (A3) together with a corresponding decrease in PAHs containing one anthracene skeleton (A2) is observed for the Py extracts of the two coals. Coal extraction in CHCl3 is relative mild, and it only affects the relatively small molecules of the mobile phase,9,40 but pyridine is an efficient extracting solvent that causes swelling of the vitrinite and the consequent removal of larger PAHs. The swelling phenomenon permits labile bonds between PAHs with four or more condensed rings and a breakdown in the network. For Mieres W coal, a slightly higher proportion of PAHs with two or three condensed rings (naphthalene skeleton) in the Py extracts is observed (Figure 3a). Mieres coal is moderately more mature than Nalon coal and has started to generate hydrocarbons that open the (40) Van Krevelen, D. W. Coal. Typology-Chemistry-Physics-Constitution; Elsevier: Amsterdam, 1961 (1993, complementary revised edition).

Figure 3. Distribution of different families of PAHs in chloroform and pyridine extracts of the wet coals: A1, two or three lineally condensed aromatic rings; A2, three or four lineally condensed aromatic rings; A3, four or more lineally condensed aromatic rings.

pore structure. Extraction with pyridine produces two parallel effects: first, swelling of the coal which favors the extraction of relatively larger PAHs with four or more condensed rings and those with similar fluorescence properties (A3), and second, a rupture of labile bonds which leads to a change in the coal matrix and, consequently, a slight increase in the proportion of the

954


Figure 4. Distribution of different families of PAHs in the chloroform and pyridine extracts of preheated coals: A1, two or three lineally condensed aromatic rings; A2, three or four lineally condensed aromatic rings; A3, four or more lineally condensed aromatic rings.

extractable light aromatic compounds with two or three rings (A1, one naphthalene nucleus). For Nalon coal, the first effect dominates. Nalon coal is slightly less mature, and it still retains its oil potential which is a function of the quantities of nonpolyaromatic hydrocarbon structures.41 For this coal, swelling induced by pyridine leads to the release of a higher proportion of hydrocarbons of type A3 present in the extract, and consequently, a relatively lower proportion of relatively smaller PAHs with one naphthalene or anthracene nucleus (A1 and A2 in Figure 3b) compared to the CHCl3 extract. Comparison between Mieres and Nalon Preheated Coals. Comparison of the distribution of different classes of PAHs in CHCl3 and Py extracts of Mieres and Nalon preheated coals shows that the Mieres coal has a relatively higher level of maturation (Table 4). Mieres P coal extract has a higher proportion of larger PAHs than the Nalon P coal extract (14.4 vs 10%). As previously discussed for wet coals, pyridine also produces a relative increase in the amount of larger PAHs (A3) and a corresponding decrease in the proportion of PAHs with three or four lineally condensed rings (A2) in the extracts of the two coals (Figure 4). In addition, a decrease of lighter PAHs with two or three aromatic rings is also observed. The maximum intensity of the first two SF peaks shifts toward slightly longer wavelengths for Mieres P coal (353 and 395 nm) (41) Durand, B.; Monin, J. C. Kerogen. Insoluble Organic Matter from Sedimentary Rocks; Durand, B., Ed.; Editions Technip: Paris, 1973; Chapter 4, pp 113-142.

Kister et al.

than for Nalon P coal (345 and 393 nm, respectively), according to the rank of the unheated coal (Table 5). This indicates a slight increase in the average molecular weight of these families of compounds. It should be pointed out that the difference in λmax of the second peak is very close to the experimental error (1 nm). Effects of Coal Preheating. A previous study reported effects of preheating on thermodesorption, involving a loss of hydrocarbons of small molecular size.16 To appreciate more fully the structural differences caused by the preheating process, a comparative study of the coal extracts (by SF analysis) and the parent coal (by FTIR) was made. Table 6 shows the values of several indices calculated either from the area or the absorbance of appropiate absorption bands. Four different aromaticity indices (V, AO, AC, and AD) are defined to show the changes that took place in the aliphatic/aromatic groups. Both indices V and AO are based on the aromatic and aliphatic hydrogens. The aliphatic hydrogen was estimated in the region 29802750 cm-1 (aliphatic C-H stretching vibrations). To estimate the concentration of the aromatic hydrogen, the area of the absorption band due to aromatic C-H stretching vibrations at 3050 cm-1 (Area3100-2980) was used to obtain the V index. For the AO index, the area of the bands between 900 and 700 cm-1 (out-of-plane vibration of aromatic C-H bonds) was considered. The other two aromaticity indices (AC and AD) related the absorbance of the band centered at 1600 cm-1 (H1600), due to aromatic CdC, with those of the aliphatic groups (H2920 + H2855) (AC index) and the absorbance of methyl (1375 cm-1) and methylene (1440 cm-1) bending modes (AD index). Finally, two other indices were defined: first, a methyl index, AB, which reflects the relative variation of methyl and methylene groups (H1375/H1440), and second, the AE index defined as (H2920 + H2855)/ (H1600 + H2920 + H2855). This index is similar to the factor defined by Ganz and Kalkreuth42 and can be used in a way similar to the traditional H/C-O/C van Krevelen diagram to distinguish between coals of different rank and to define the oil potential of a given coal. Figure 5 shows an example of the FTIR spectra of Mieres coals, wet (W) and preheated (P). All the areas used in the calculation of the indices are shown in Figure 5 together with the intensity of the above absorption bands (H) determined using the baseline method. The decrease in the H/C atomic ratio due to preheating that is shown in the van Krevelen diagram (Figure 6) can be attributed to the rupture of aliphatic chains and a loss of hydrocarbons. Furthermore, although the accuracy of this measurement is not sufficient for detailed interpretation, of particular significance is the shift of the absorption FTIR band at 1600 cm-1 (attributable to aromatic CdC) toward slightly lower wavenumbers. Such a shift suggests a more slightly extended polycondensation for Nalon than Mieres preheated coals (Table 6). In addition, the increase in the aromaticity indices (V, AO, AC, and AD indices) derived from FTIR data corroborates the loss of aliphatic groups (Table 6). This is confirmed by the relative increase in methyl groups for Mieres and Nalon preheated coals, which is reflected by the increase in the AB index. With preheating, the oil potential index (AE index) decreases (42) Ganz, H.; Kalkreuth, W. Fuel 1987, 66, 708-711.



Table 6. FTIR Structural Indices for Mieres and Nalon Coals (W, P, Wox, and Pox) FTIR indicesa V

AO

wet (W) preheated (P) wet oxidized (Wox) preheated oxidized (Pox)

0.110 0.122 0.127 0.104

0.292 0.587 0.541 0.476

wet (W) preheated (P) wet oxidized (Wox) preheated oxidized (Pox)

0.085 0.089 0.088 0.085

0.397 0.490 0.472 0.302

AC

AD

AB

AE

position ν(CdC) (cm-1)

Mieres Coal 0.930 0.493 1.146 0.507 1.076 0.507 1.006 0.524

0.691 0.725 0.756 0.712

0.518 0.466 0.482 0.498

1601 1598 1596 1598

Nalon Coal 1.178 0.531 1.355 0.558 1.362 0.546 1.225 0.607

0.720 0.733 0.774 0.740

0.459 0.425 0.423 0.449

1601 1596 1596 1597

a Definition of FTIR indices: aromaticity indices, V ) Area 3100-2980/Area2980-2750, AO ) Area900-710/Area2980-2750, AC ) H1600/(H2920 + H2855), AD ) H1600/(H1440 + H1375); methyl index, AB ) H1375/H1440; oil potential index, AE ) (H2920 + H2855)/(H1600 + H2920 + H2855). The relative standard deviations (RSDs) for five different samples were 5% for the V index, 4% for the AO index, 3% for the AC index, and 2% for the AD, AB, and AE indices.

Figure 5. FTIR spectra of Mieres coals: (a) wet (W) and (b) preheated (P). H: maximum absorbance of the corresponding band. Assignments of these absorption bands are summarized in Table 2.

Figure 6. Evolution path in a van Krevelen diagram for Mieres and Nalon coals: W, wet coal; P, preheated coal; Wox, wet oxidized coal; Pox, preheated oxidized coal.

by about 10% for Mieres and 7% for Nalon coal, suggesting that Mieres coal is more affected by preheating (Table 6). In conclusion, it appears that upon preheating aliphatic groups are liberated while at the same time the macromolecular network increases in aromaticity. Effects of preheating can also be observed in the SF spectra by comparing distributions of different classes of PAHs in the CHCl3 and Py extracts (Table 4). In general, the proportion of each family in the CHCl3 extracts is essentially the same for the Nalon wet and

preheated coals, while Mieres coals, wet and preheated, show small differences. However, where pyridine is used, differences between the wet and preheated coal become more significant in Mieres coal extracts. Pyridine increases the release of larger PAHs of type A3 which are weakly bound to the network. However, there is no indication of any significant change in Nalon coal (Table 4). In this case, preheating produces a relatively strong interaction between PAHs with two or three rings and those with four or more condensed rings which cannot be extracted using pyridine. These findings are in agreement with a shift of the FTIR band at 1600 cm-1 toward lower frequencies (Table 6). The variation of the fluorescence indices for the extracts in CHCl3 and pyridine of wet and preheated coals clearly reflects the different behaviors of Mieres and Nalon wet coals when subjected to the preheating process (Figure 7).It is important to note that alkylsubstituted PAHs lead to a shift to a longer wavelength of the maximum peak intensity (bathochromic effect) in relation to the corresponding unsubstituted PAHs. This effect is more significant for smaller molecules than for larger polyaromatic molecules and is slight for methyl substitution.23,24 Furthermore, peak shifts to longer wavelengths are also produced by the presence of oxygenated groups (ethers, carboxylic acids, phenols, etc). Consequently, the higher or lower proportion of alkyl-substituted and/or oxygenated substituted PAHs could lead to bathochromic (shift to a longer wavelength) or hypsochromic (shift to a shorter wavelength) effects, respectively. Since preheating produces a loss of aliphatic groups, the shift toward a shorter wavelength (Mieres W, 359 and 402 nm, and Mieres P, 353 and 396 nm, respectively) can easily be explained (Table 5). This, perhaps, is a consequence of a lower proportion of PAHs with alkyl substituents in the coal preheated extracts. Although smaller hypsochromic shifts are observed for the Nalon coal extracts (Nalon W, 346 and 395 nm, and Nalon P, 345 and 393 nm, respectively), probably due to the lower rank of this coal, they are not significant enough to be considered. Decarboxylation may also occur upon preheating (O/C atomic ratio decreases, cf. Figure 6), producing a hypsochromic effect. Effects of Oxidation on Wet Coals. The distribution of PAH families of CHCl3 extracts of wet oxidized (Wox) and wet (W) coals is quite similar. However, considering the fluorescence indices for the CHCl3 extracts of these coals and taking into account that differences are quite small and near to the experimental

956


Kister et al.

Figure 9. Variation of the fluorescence indices for the CHCl3 extracts (In(Pox)-In(P), where n ) 1, 2, 3, or 4): P, preheated coal; Pox, preheated oxidized coal.

Figure 7. Variation of the fluorescence indices for (a) the CHCl3 extracts and (b) the pyridine extracts (In(P)-In(W) and Rn(P)-Rn(W), where n ) 1, 2, 3, or 4): W, wet coal; P, preheated coal.

Figure 8. Variation of the fluorescence indices for the CHCl3 extracts (In(Wox)-In(W), where n ) 1, 2, 3, or 4): W, wet coal; Wox, wet oxidized coal.

error, a different behavior on oxidation can be observed for Mieres and Nalon coals (Figure 8). The findings are compatible with those drawn from FTIR data and the diagram of the H/C-O/C atomic ratios. Oxidation of wet coals produces a decrease in the H/C atomic ratio and an increase in the O/C atomic ratio as can be observed in the van Krevelen diagram (Figure 6). On the basis of the FTIR data, oxidation of wet coals produces a shift of the band at 1600 cm-1. In the case of Mieres coal, the degree of condensation is slightly higher than that produced by preheating, while an equivalent effect is observed for Nalon coal (Table 6). The increase in the degree of condensation is in agreement with a slight increase in the relative proportion of the PAHs with two or three condensed rings in

detriment to the larger PAHs (Table 4). Oxidation produces a loss of aliphatic groups (hypsochromic effect) and an increase in the oxygenated functionality (bathochromic effect). The two above phenomena and those that involve the incorporation of some molecules to the macromolecular network produced a slight shift of the wavelengths of maximum peak intensity (Table 5) for Mieres coal (359 and 402 nm vs 356 and 400 nm, respectively, for Wox). Similarly, FTIR indices that reflect hydrogen aromaticity (V, AO, AC, and AD indices) increase with oxidation for both coals (Table 6). On the other hand, a decrease in the oil potential (AE index) can be observed in both cases (Table 6), indicating the negative character of this reaction.9 This decrease can be directly related to the lower CHCl3 extract yield and, in particular, to the relative proportion of larger PAHs. These observations can be corroborated by the relative increase in the AB index attributed to a loss of long aliphatic chains (Table 6). Thus, oxidation leads to a consumption of the aliphatic groups and the formation of oxygen bridges and oxygen functional groups such as carboxyl and carbonyl groups. This may be related to the modification of the reticulation of the solid matrix that caused the decrease in the oil potential. Consequently, it is more difficult to remove the hydrocarbons by CHCl3 extraction. As a result of a partial reticulation phenomenon, the liquid phase is trapped in the solid phase and a relatively small proportion of the large size PAHs can be linked to the network, increasing its aromaticity. Effects of Oxidation on Preheated Coals. Different trends can be observed as a consequence of the two successive reactions, preheating and oxidation. The same considerations made for wet coals can be established. A more significant variation of the proportion of PAHs with two or three condensed rings (one naphthalene skeleton) can be deduced from the variation of fluorescence indices I1 and I3 (Figure 9).A different behavior toward oxidation of preheated coals can be also deduced from the van Krevelen diagram (Figure 6) in comparison to wet coals. There is an increase in the O/C atomic ratio due to oxygen fixation accompanied by an increase in the H/C ratio. From the FTIR data, oxidation has no effect on the shift of the absorption band at 1600 cm-1. The aromaticity indices (V, AO, and AC) decrease and the oil potential index (AE index) increases. However, the methyl index (AB index) decreases for Mieres Pox coal but increases for Nalon



Pox coal (Table 6). The results from fluorescence spectroscopy confirm those derived from FTIR data, in particular for the I1 index which decreases during oxidation of the wet coals (Figure 8) and the Mieres preheated coal (Figure 9). On the other hand, it increases for the Nalon preheated coal (Figure 9). Preheating causes a loss of hydrocarbons and, consequently, a consumption of aliphatic chains which reduces the number of the most active sites exposed to oxidation. Because Mieres coal appears to be more affected by preheating, a portion of hydrocarbons is trapped in the macromolecular network, which might be more easily affected by oxidation due to a consumption of the aliphatic chains and the formation of oxygenlinked bridges. Consequently, the fluorescence index I1 decreases for Mieres Pox coal, suggesting that the behavior is similar to that of Mieres Wox. On the other hand, the structural changes induced by preheating in Nalon coal provide fewer active sites vulnerable to oxidation, allowing the consumption of the rest of aliphatic chains together with the reticulation phenomenon of the solid phase. Thus, removal of the lightest hydrocarbons increases but the degree of condensation remains the same. Concerning the shift of the maximum wavelength of the fluorescence peaks, two opposite effects must be considered: first, the formation of oxygenated aromatic compounds indicated by the bathochromic shift, and second, an hypsochromic effect due to shorter chains attached to aromatic rings. This is reflected by the stabilization of the maximum intensity of the second fluorescence peak for Mieres P and Pox coals (396 nm).

maturation, giving rise to opposing trends in the fluorescence index I1. The I1 index appears to be a good indicator of reactivity and indicates the possibility of aromatization and condensation together with the incorporation of some molecules to the macromolecular network. The effects of oxidation are more complex, and two opposed phenomena can occur simultaneously, making the interpretation of small variations difficult: a hypsochromic shift due to the shortening of the alkyl substituents of the aromatic rings or a bathochromic shift due to the quenching effect caused by the oxygenated substituents. Nevertheless, the results of oxidation of wet and preheated coals might suggest condensation, a loss of aliphatic chains, and a reticulation of the macromolecular network due to the formation of oxygenated compounds. The comparisons made between the CHCl3 and pyridine extracts provide evidence of the molecules trapped in, or weakly bonded to, the macromolecular network of the wet coals, enabling the assessment of the different maturation level of the starting coal (wet) and determination of the effects of coal preheating. In all cases the fluorescence data can be taken as an indirect parameter of coal rank, which is not altered by the reactions that take place (preheating and oxidation, individual or successive reactions). Thus, fluorescence analyses are suitable for giving a rapid overview of the structural changes caused by preheating. The observations made by fluorescence are confirmed by the FTIR indices of the whole coals. Further studies are being carried out on a wide range of international coals used in the coking industry to assess the relation, if any, between structural changes induced by preheating and coking properties.

Conclusions Synchronous UV fluorescence spectroscopy provides useful information on the distribution of different classes of PAHs, taking into account the relative abundance of each PAH family, the different indices calculated from the area, and the height of the fluorescence peaks and their maximum wavelength. Preheating of coal can be explained by thermodesorption which causes a loss of the oil potential according to the level of

Acknowledgment. The authors thank the DGICYT (Projects Nos. PB90-0067 and PB90-0104) for financial support. Prof. Tuan Vo-Dinh, OAK Ridge National Laboratory (Tennessee), is thanked for his interest and his help, especially for the many useful comments that have improved the manuscript. EF950159A

Effects of Preheating and Oxidation on Two Bituminous Coals

Recommend Documents