Investigating the Formation Mechanism of Soot-like Materials Present

Energy & Fuels 2008, 22, 3317–3325

3317

Investigating the Formation Mechanism of Soot-like Materials Present in Blast Furnace Coke Samples ´ lvarez,‡ N. Paterson,*,† D. R. Dugwell,† and R. Kandiyoti† S. Dong,† P. A Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom, and Instituto Nacional del Carbo´n, C/Francisco Pintado Fe, 26, 33011 OViedo, Spain ReceiVed June 16, 2008. ReVised Manuscript ReceiVed July 17, 2008

“Soot-like” materials, extractable by 1-methyl-2-pyrrolidinone (NMP), have been found in cokes from the lower temperature regions of a blast furnace. Size-exclusion chromatography (SEC) suggests that apparent molecular masses of these materials are very large. An attempt to gain an understanding of the formation mechanism of these “soot-like” materials has been made by means of tracing the changes in the molecularmass distribution and molecular structure of the NMP-extractable materials from an injectant coal as well as its partially gasified chars and its pyrolytic tars. Variations in the SEC chromatograms provide clues about changes in the apparent molecular-mass distributions of these NMP extracts. Results suggest that the build-up of “soot-like” materials follows from the secondary reactions of tars evolved from the injectant coal. The likely secondary-reaction pathways have been probed by collating structural information on these NMP extracts. The time-resolved 13-16 and 22-25 min elution fractions from the SEC column have been characterized using UV fluorescence (UV F) spectroscopy. Greater concentrations of larger aromatic ring systems are found present in samples formed under conditions appearing more prone for soot formation. The 11-16 min (large apparent molecular mass) effluent from SEC has been examined by Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM). Results from FTIR spectroscopy are consistent with the UV F data, showing more significant extents of dehydrogenation under conditions more prone to form soot. Similarly, TEM results show that larger amount of graphene layers exist in samples exposed to more soot-prone conditions. The emerging picture for the formation of “soot-like” materials involves a welldefined sequence. Tars evolved from the injectant coal undergo secondary dehydrogenation, condensation, and reploymerization reactions, which eventually lead to the formation of the NMP-extractable “soot-like” materials of large apparent molecular mass.

Introduction Pulverized coal injection into blast furnaces has become standard practice in most major steelworks. Injection rates usually vary from 100 to over 200 kg coal per ton of hot metal (kg/thm). Operation with low coal injection rates has been reported to give a positive effect on the overall blast furnace efficiency.1 However, when the coal injection rate exceeds approximately 200-225 kg per ton of hot metal, operational problems have been encountered. These include substantial increases in the level of dust emission, reduction in the permeability of the coke bed, instability of the raceways, and poor drainage of molten slag and iron through the bosh and deadman into the furnace hearth.2 To overcome these problems and further increase the injection rate, it is necessary to have a good knowledge of the fate of coal inside the blast furnace. A number of studies have been carried out at Imperial College to obtain some insights into the fate of injectant coal and its * To whom correspondence should be addressed. Telephone: +44-(0)207594-5634. Fax: +44-(0)20-7594-5638. E-mail: [email protected]. † Imperial College London. ‡ Instituto Nacional del Carbo ´ n. (1) Yamaguchi, K.; Ueno, H.; Tamura, K. Maximum injection rate of pulverized coal into blast furnace through tuyeres with consideration of unburnt char. Iron Steel Inst. Jpn. Int. 1992, 32 (6), 716–724. (2) Bortz, S.; Flament, G. Experiments on pulverized coal combustion under conditions simulating a blast furnace environment. Ironmaking Steelmaking 1983, 10 (5), 222–229.

effect on the performance of blast furnaces. The pyrolysis, combustion, and gasification behavior of injectant coal particles under conditions simulating those in the blowpipes, tuyeres, and raceways of blast furnaces has been studied in bench-scale reactors.3,4 One of the key conclusions from these studies is that coal char is unlikely to combust to a high extent, as a result of the low concentration of O2 available to coal chars and the short residence time of chars at peak temperatures.4 In addition, Dong et al.5,6 have characterized a suite of tuyere-level coke samples taken directly from working blast furnaces. Materials of polystyrene-equivalent molecular mass, in excess of 107 units, have been found present in relatively lower temperature regions of the blast furnace, i.e., the bosh, the rear of the bird’s nest, and the deadman. These materials eluted from a Mixed-A column on a size-exclusion chromatography (SEC) system from (3) Pipatmanomai, S.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Investigation of coal conversion under conditions simulating the raceway of a blast furnace using pulsed air injection, wire-mesh reactor. Energy Fuels 2003, 17 (2), 489–497. (4) Wu, L.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Simulation of blast furnace raceway conditions in a wire-mesh reactor: Interference by the reactions of molybdenum mesh and initial results. Energy Fuels 2006, 20 (6), 2572–2579. (5) Dong, S.; Wu, L.; Paterson, N.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Investigating the fate of injectant coal in blast furnaces by size-exclusion chromatography. Energy Fuels 2007, 21 (2), 1062–1070. (6) Dong, S.; Paterson, N.; Kazarian, S. G.; Dugwell, D. R.; Kandiyoti, R. Characterization of tuyere-level core-drill coke samples from blast furnace operation. Energy Fuels 2007, 21 (6), 3446–30454.

10.1021/ef800466h CCC: $40.75  2008 American Chemical Society Published on Web 08/16/2008

3318 Energy & Fuels, Vol. 22, No. 5, 2008

11 to ca. 13 min, shorter than the exclusion limit of the system (ca. 14 min). Primary soot particles from fuel-rich combustion systems have been found and examined in several other studies.7-14 Evidence from various analytical techniques indicate that diameters of primary soot particles, formed in combustion flames, range from 20 to 60 nm.7-9 In our previous calibration work15 using a set of three-dimensional standards, soot particles with a nominal diameter of 40 nm have been found to elute from the SEC column at a time very close to 11 min; colloidal silica particles with a nominal diameter of 10 nm would elute at about 12 min. Apicella et al.16 also found that a fraction of ethylene flame soot, insoluble in dichloromethane, eluted as a sharp single peak centered at ca. 11 min from the same SEC column. Other researchers have also found that soot particles from ethylene combustion have diameters of about 60 nm.10 Primary soot particles from diesel engines range in size between 18 and 33 nm.11 Soot particles of diameter about 40-50 nm have been detected in the dust from blast furnace operation with coal injection.13 Furthermore, it is noted that the lower temperature regions of the blast furnaces were lean in oxygen and rich in carbon and, thus, represented a soot-prone environment. It therefore seems reasonable to view as “soot-like” the 11-13 min eluting materials of large apparent molecular mass, recovered from the extraction of coke samples from a blast furnace. Soot is important to the performance of blast furnace with coal injection, in part because of its radiant heat-transfer effects. There is much less soot, in terms of mass, present in a coal flame than other solid particles (e.g., char, ash, etc.). However, the small size of soot particles results in a large total surface area that leads to significant heat transfer from the flame to the surroundings via radiation (from the soot particles). Radiation tends to lower flame temperatures by several hundred degrees, and the temperatures of surrounding gas and the furnace wall are likely to be increased.17,18 The variation in temperature also affects the formation of NOx, which has a huge impact on the (7) Hessler, J. P.; Seifert, S.; Winans, R. E. Small-angle X-ray scattering for studies of soot inception and formation. Prepr. Pap.-Amer. Chem. Soc., DiV. Fuel Chem. 2002, 47 (2), 736–737. (8) Choi, K.; Mochida, I. Characterization of particulate matter emitted from diesel engine. Prepr. Pap.-Amer. Chem. Soc., DiV. Fuel Chem. 2002, 47 (2), 776–777. (9) Blevins, L. G.; Yang, N. Y. C.; Mulholland, G. W.; Davis, R. W.; Steel, E. B. Early soot from inverse diffusion flames. Prepr. Pap.-Amer. Chem. Soc., DiV. Fuel Chem. 2002, 47 (2), 740–741. (10) Glasier, G. F.; Pacey, P. D. Observation and collection of an aerosol in ethane at 1184 K. Prepr. Pap.-Amer. Chem. Soc., DiV. Fuel Chem. 2002, 47 (2), 742–743. (11) Kim, S. H.; Fletcher, R. A.; Zachariah, M. R. Understanding the difference in oxidative properties between flame and diesel soot nanoparticles: The role of metals. EnViron. Sci. Technol. 2005, 39 (11), 4021– 4026. (12) Mathis, U.; Mohr, M.; Kaegi, R.; Bertola, A.; Boulouchos, K. Influence of diesel engine combustion parameters on primary soot particle diameter. EnViron. Sci. Technol. 2005, 39 (6), 1887–1892. (13) Apicella, B.; Barbella, R.; Ciajolo, A.; Tregrossi, A. Comparative analysis of the structure of carbon materials relevant in combustion. Chemosphere 2003, 5, 1063–1069. (14) Pipatmanomai, S.; Herod, A. A.; Morgan, T. J.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. The fate of volatiles from injectant coals and soot formation in blast furnaces. Energy Fuels 2004, 18, 68–76. (15) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. The calibration of size exclusion chromatography columns: Molecular mass distribution of heavy hydrocarbon liquids. Energy Fuels 2004, 18, 778–788. (16) Apicella, B.; CIajolo, A.; Suelves, I.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Structural characterization of products from fuel-rich combustion: An approach based on size exclusion chromatography. Combust. Sci. Technol. 2002, 174 (11 and 12), 345–359. (17) Im, K. H.; Ahluwalia, R. K. Int. J. Heat Mass Transfer 1993, 36, 293–302.

Dong et al.

environment. In blast furnaces, soot is carried by the gas flow into the coke bed and may contribute to the loss of permeability of the coke bed. Soot formation in coal flame is complex and might follow a different pathway from that of soot formation from simple hydrocarbon fuels. This is because the molecules of volatiles, particularly tar molecules, evolved from coal during pyrolysis, are much larger and more chemically diverse than those of simple hydrocarbon fuels. Tars are expected to undergo secondary reactions because of the high temperature present in many coal use processes. Secondary reactions of tars from primary pyrolysis have been widely reported. Doolan et al.19 used quartz tubular reactors to study secondary cracking of the tar vapors generated in a fluidized bed pyrolyzer. Substantial cracking was found to occur beyond a certain temperature, 870 K in their case. No CO2 was produced from the cracking of the tars. Gases, predominantly alkenes and smaller yields of benzene and methane, plus a liquid product defined as secondary tars, and a solid product believed to be soot, were observed in these experiments. Freihaut et al.20 also found significant quantities of secondary decomposition product gases (HCN, acetylene, and CO) in their study of secondary cracking of tar released from a bituminous coal in a flash lamp reactor. Nenniger et al.21 studied the sooting potential of several coals by separating aerosol from char particles after the pyrolysis of the coals in argon in a laminar flow furnace. The aerosol was believed to consist of extractable tar, soot, and condensed ash. As the pyrolysis temperature was raised, they found that the soot yield increased but the tar yield decreased. However, the sum of soot plus tars remained approximately constant despite the increasing severity of pyrolysis. At the highest temperature tested (2200 K), about 20% by weight of a dry high-volatile bituminous coal was converted to soot. Wornat et al.22 also reported that about 20% by weight of a high-volatile bituminous coal was converted to soot at high temperatures and long residence times. In addition, the sum of polycyclic aromatic compounds (PACs) recovered and soot yields was relatively constant (supporting the findings of Nenniger et al.21). This suggests that PAC served as precursor to soot. They also investigated changes in the composition of these PACs and found that a loss in compositional complexity was observed as the severity of secondary reactions increased, implying the selective survival of a group of more stable species. They further pointed out that compounds with more complex attachments were more reactive than those with simple or no attachments. Chen and co-workers23 performed coal pyrolysis experiments in argon in an inductively heated radiant drop-tube furnace. They (18) Ribgy, J.; Ma, J.; Webb, B. W.; Fletcher, T. H. Transformations of coal-derived soot at elevated temperature. Energy Fuels 2001, 15 (1), 52–59. (19) Doolan, K. R.; Mackie, J. C.; Tyler, R. J. Coal flash pyrolysis: Secondary cracking of tar vapours in the range 870-2000 K. Fuel 1987, 66 (4), 572–578. (20) Freihaut, J. D.; Proscia, W. M.; Mackie, J. C. Chemical and thermaochemical properties of heavy molecular weight hydrocarbons evolved during rapid heating of coal of varying rank characteristics. Combust. Sci. Technol. 1993, 93 (1), 323–247. (21) Nenniger, R. D.; Howard, J. B.; Sarofim, A. F. Sooting potential of coals. International Conference on Coal Science, Pittsburgh, PA, 1983; p 521. (22) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Changes in the degree of substitution of polycyclic aromatic compounds from pyrolysis of a highvolatile bituminous coal. Energy Fuels 1987, 1 (5), 431–437. (23) Chen, J. C.; Niksa, S. In Proceedings of 24th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1992; p 1269.

Soot-like Materials in Blast Furnace Coke Samples

Energy & Fuels, Vol. 22, No. 5, 2008 3319

Table 1. Elemental and Proximate Analyses of EC2386 Coal coal EC2386

ash volatile C H N S O (% db) matter (% db) (% db) (% db) (% db) (% db) (% db) 7.9

13.9

83.7

3.73

1.49

0.34

2.5

also reported that the yields of tar/oil plus soot in the secondary pyrolysis experiments were constant and almost equaled to the tar-plus-oil yields obtained at the longest residence time in primary pyrolysis experiments. For a high-volatile bituminous coal at higher temperatures, more than 25% of the coal mass (daf) was converted to soot. For the gas-phase products, C3 hydrocarbons and ethane decreased monotonically with temperature, while methane and ethylene reached a maximum and then dropped. Acetylene increased dramatically during soot formation and growth. The present study aims to develop a better understanding of the formation of “soot-like” materials found in blast furnaces. This was performed by tracing structural changes of 1-methyl2-pyrrolidinone (NMP)-extractable materials adhering to cokes and chars within the blast furnace. The changes provide clues about the formation mechanisms of “soot-like” materials as well as providing complementary information on the manner in which coal is consumed in a blast furnace. Experimental Section Selection of Samples. Two char samples (designated as CGR179 and CGR181; CGR stands for the “coal gasification rig”) were prepared by partially gasifying an injectant coal (designated as EC2386, see Table 1 for its elemental analysis) in the coal gasification rig (CGR) at the Teesside Technology Centre of Corus U.K. Ltd. The schematic diagram of the rig is shown in Figure 1. GR179 was prepared by gasifying EC2386 coal at 1700 °C in air, whereas CGR181 was prepared in oxygen-enriched air (30% oxygen) but under otherwise the same conditions as CGR 179. Both the air and oxygen-enriched air were preheated to 1098 °C by a plasma heater. The velocity of the blast resulted in a residence time of about 27 ms, typical of that in the raceway. The gasification weight losses under these two conditions are given in Table 2 by Corus Ltd. EC2386 coal was also pyrolyzed in a wire mesh reactor (WMR) at Imperial College London at 1.0 bara with a heating rate of 1000 °C/s to a peak temperature of 1700 °C and held at the peak temperature for 0 s to prepare the char and tars. The total volatile yield from the WMR experiment is also given in Table 2. A detailed description of the wire mesh reactor can be found elsewhere.24 Because of the configuration of the wire mesh reactor, tars, evolved during pyrolysis, were swept immediately away by the purging helium from the hot zone into the tar trap cooled by liquid nitrogen.24,25 As a result, the secondary reactions of tars and the deposition of secondary-reaction products onto the char particles were minimized.24,25 The EC2386 WMR char is thus considered to be “clean”, i.e., nearly free from the deposition of secondary-reacted tars. In contrast, the two CGR chars are likely to retain some secondary-reaction products, because of the fact that the developing char particles were surrounded by a plume of primary tars at high temperatures in the rig. Primary tars, consisting of many polycyclic aromatic hydrocarbons (PAHs), were very likely to have undergone secondary reactions if tarrying in high-temperature regions. In the presence of oxygen, part of the secondary products was converted into CO, CO2 and H2O. Nevertheless, deposition of some secondary products onto the char particles is expected to have occurred. (24) Kandiyoti, R.; Herod, A.; Bartle, K. Solid Fuels and HeaVy Hydrocarbon Liquids: Thermal Characterization and Analysis; Elsevier Science Publisher: Amsterdam, The Netherlands, 2006. (25) Gibbins, J. R. Ph.D. Thesis, University of London, London, U.K., 1988.

In addition, a coke sample (designated as Core1), taken from the bosh region of a working blast furnace, has also been analyzed as a reference sample representative of the very soot-prone environment in the blast furnace. This sample has been used in our previous study of the carbon structure of a suite of tuyerelevel coke samples extracted from the same blast furnace.6 Extraction of Samples. The chars and EC2386 coal were extracted by 1-methyl-2-pyrrolidinone (NMP) for 1.5 h in an ultrasonic bath. NMP was used as the solvent because of its ability to achieve high extract yields under relatively mild conditions. The solid/solvent ratio was 1 g/600 mL. This ratio allows for a high extent of extraction. Only a very small amount of materials could be further extracted, as indicated by almost transparent NMP solution of the second extraction. The extraction solution was then passed through a 1 µm glass fiber filter paper. The filter paper together with the filter cake was then washed several times with a 4:1 acetone/deionized water mixture to remove remaining NMP. The filter paper and the residues were dried in a vacuum oven overnight at 80 °C. The extractable yields, calculated by eq 1, are shown in Table 3.

% extractable yield ) (weight of original sample - weight of residue) × 100% (1) C content of original sample Size-Exclusion Chromatography. The apparent molecular-mass distribution of the NMP extracts from EC2386 coal, its chars, and the bosh coke Core1 has been obtained using the size-exclusion chromatography equipped with a Mixed-A column (7.5 mm diameter × 300 mm length) from Polymer Laboratories U.K. The column was packed with polystyrene-polydivinylbenzene beads of 20 µm nominal diameter. NMP was used as the eluent, because it has been shown to elute molecules of most coal-derived liquids by a predominantly size-exclusion mechanism and with minimum surface effects in contrast to other solvents, such as pyridine or tetrahydrofuran (THF),26-28 where surface interaction effects are more important. More evidence of drawbacks to THF as the eluent are introduced elsewhere.29 The column was operated at room temperature with a flow rate of 0.5 mL/min. Two detectors were used: an evaporative light scattering (ELS) detector (flow rate of 0.8 L/min at 4 bara; nebulizer and evaporator temperatures of 210 and 150 °C, respectively) and a diode-array UV-vis absorption detector with multiple wavelengths (280, 300, 350, 370, and 450 nm). The Mixed-A column was calibrated using a wide range of polymer standards: polystyrene (PS) up to 15.5 × 106 units (units is Da or amu), polymethylmethacrylate (PMMA) up to ∼1 × 106 units, and polysaccharide (PSAC) up to 788 000 units.15,30 The calibration was checked for accuracy by the following procedures: (a) The elution times of model compounds up to slightly above 1000 units were compared to the polystyrene calibration. Structural features of these compounds ranged from polynuclear aromatics to dyes and organic acids.15 (b) Mass determinations up to m/z 3000, by matrix-assisted laser desorption/ionization-mass spectroscopy

(26) Johnson, B. R.; Bartle, K. D.; Cocksedge, M.; Herod, A. A.; Kandiyoti, R. Absolute calibration of the size-exclusion chromatography of coal derived materials through matrix assisted laser desorption/ionisation mass spectrometry. Fuel 1998, 77, 933–945. (27) Herod, A. A.; Zhang, S. F.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Solubility limitations in the determination of molecular mass distributions of coal liquefaction and hydrocracking products: N-Methyl 2-pyrrolidinone as mobile phase in size exclusion chromatography. Energy Fuels 1996, 10, 743–750. (28) Lafleur, A. L.; Nakagawa, Y. Multimode size exclusion chromatography with poly(divinylbenzene) columns and N-methylpyrrolidinone for the characterization of coal-derived mixture. Fuel 1989, 68, 741–752. (29) Herod, A. A.; Bartle, K. D.; Kandiyoti, R. Characterization of heavy hydrocarbons by chromatographic and mass spectrometric methods: An overview. Energy Fuels 2007, 21, 2176–2203. (30) Herod, A. A.; Zhuo, Y. Q.; Kandiyoti, R. Size-exclusion chromatography of large molecules from coal liquids, petroleum residues, soots, biomass tars and humic substances. J. Biochem. Biophys. Methods 2003, 56 (1-3), 335–361.


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Figure 1. Schematic diagram of the coal gasification rig (CGR) (left) and the setup of the rig (right) at the Teesside Technology Centre of Corus U.K. Ltd. Table 2. Pyrolysis and Oxidation Weight Loss of Char Samples Prepared in CGR and WMR char

total weight lossa (% db)

pyrolysis weight loss (% db)

weight loss upon oxidation (% db)

residence time (ms)

O/C ratio

oxygen level (%)

CGR179 CGR181 WMR char

27 36 34.7

8 10 34.7b

19 26 0

27 27 0c

6.4 9.2 0

20.9 30 0

a Total weight loss ) pyrolysis weight loss + weight loss upon oxidation. b Averaged data for the WMR char; the standard deviation is 0.62. For the WMR run, the pyrolysis weight loss is the total volatile yield, defined as: pyrolysis weight loss (%) ) total volatile yield ) [(weight of original sample - weight of residue) × 100%]/weight of original sample. c The hold time at peak temperature (1700 °C) was 0 s; the heating-up lasted for ca. 1.7 s.

Table 3. Extraction Yields of EC2386 Coal and Its Chars sample

average extraction yield (wt % carbon content)

standard deviation

EC2386 coal CGR179 CGR181 WMR char

33.7 18.9 17.6 1.3

0.41 0.78 0.82 0.36

(MALDI-MS), of fractions of pitch from the SEC,15,31 were comparable to the PS-equivalent molecular masses calculated for the measure elution times of the fractions. (c) Several polymeric standards of known molecular mass, poly-N-vinylcarbazole (90 000 units), polyvinyl acetate (170 000 units), poly(vinylpyrrolidone) (58 000 units), polyethylene-adipate) (10 000 units), and poly(vinylpyrrolidone) (3500 units),15,30 eluted close to the polystyrene calibration curve. It should be pointed out that SEC, as a non-absolute technique to estimate molecular masses, explicitly relies on the assumption of a correlation between molecular mass and molecular size. This assumption seems able to hold for the above standards tested. This is indicated by a linear relationship between the logarithms of their molecular masses and their elution time on the SEC. This relationship has been validated up to the molecular mass of 3000 units by MALDI-MS for coal-derived and similar materials.30 This assumption, however, appears uncertain for very large molecules of coal-derived materials, which eluted at very early times or “excluded” from the SEC column. Estimation of the molecular mass of the excluded materials on the basis of extrapolation of the above calibration lines gives mass values in excess of several hundred thousand universal atomic units and, at time, even in the regions of several million universal atomic units. Consequently, reservation remains to believe existence of molecules of such high masses in coal-derived and similar samples. One possible explanation emerges if the molecular conformation transits from one type to another and if this transition occurs above a certain molecular threshold adopting more three-dimensional conformations. A typical bimodal distribution of molecular mass is thus observed for coal-derived and similar samples of high polydispersity on SEC with NMP as an eluent. Herod et al.29 have discussed the bimodal distribution in great detail. They clearly pointed out that interpreta(31) Islas, C. A.; Suelves, I.; Li, W.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. The unusual properties of high mass materials from coalderived liquids. Fuel 2003, 82 (14), 1813–1823.

tion of these apparently very large masses is uncertain.29,30 Some researchers, such as Mullins et al.,32 claim that these excluded materials are aggregates of smaller molecules of, in their case, petroleum asphaltenes. They came to this conclusion after years of work with various molecular diffusion techniques and mass spectroscopy techniques. However, the possible causes for forming the aggregates are still short of detailed experimental evidence. In contrast, Herod et al.29 suggested that the excluded materials were unlikely to be aggregates after having employed salt addition, manipulating the concentration of sample solution, as well as characterizing subfractions collected at different elution intervals from the SEC column. As a result of the controversy, it is necessary to take caution in the level of confidence in SEC for estimating the molecular mass of polydispersed coal-derived and similar samples, particularly in the high mass region. Nevertheless, SEC has served well in this study to access the apparent molecular-mass distributions of samples studied. Ultraviolet Fluorescence (UV F) Spectroscopy. UV F spectroscopy has been used to characterize the lower molecular-mass fractions of the NMP extracts from EC2386 coal and its chars and tars. The spectra have been acquired with a Perkin-Elmer LS 50 luminescence spectrometer. The spectrometer features automatic correction for changes in source intensity as a function of the wavelength. The spectrometer was equipped with a transmittance accessory, and a quartz cell of 1 cm light pass length was used, with emission being detected perpendicular to excitation (90°). Synchronous spectra result from sweeping excitation and emission wavelengths with a fixed wavelength difference (20 nm in the present work) over the entire range of wavelength (i.e., from 254 to 800 nm in the current UV F instrument). Synchronous spectra have proven to give the clearest indication of the complexity of the type of samples studied here, with each type of aromatic system capable of fluorescence showing up as a separate line. In contrast, the emission and excitation spectra are normally fairly complex, even for a pure aromatic system. These spectra of currently studied samples do not differ much from each other, giving little indication of structural differences between the samples. (32) Mullins, O. C.; Martinez-Haya, B.; Marshall, A. G. Contrasting perspective on asphaltene molecular weight. This comment vs the overview of A. A. Herod, K. D. Bartle, and R. Kandiyoti. Energy Fuels 2008, 22, 1765–1773.



Figure 2. Area-normalized chromatograms of NMP extracts from EC2386 coal, its chars, WMR tar, and the bosh coke Core1 (absorption at 350 nm).

In evaluation of the synchronous UV F spectra of complex coalderived products, shifts of the maxima of the spectral lines indicate changes in the structural features of these products. If the maxima shift toward longer wavelength, the samples analyzed are thought to contain larger polynuclear aromatic ring systems. The shift is usually accompanied by a drop in signal intensity because larger ring systems have lower quantum yields.33 Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectroscopy has been used to characterize the larger molecularmass fractions of the NMP extracts from EC2386 coal and its chars. The infrared spectra have been acquired using a Nicolet Magna IR-560 spectrometer equipped with a mercury-cadmium telluride detector operating at 4 cm-1 by averaging 256 scans. The diffusion reflectance spectra were converted to the Kubelka-Munk function. Samples were well-mixed with dried KBr (sample/KBr ratio of 1:100) and then pressed into very thin pellets. Two pellets of each sample were used to derive the averaged spectra, to minimize experimental errors. Base lines were corrected in the range of 4000-1800 and 1800-400 cm-1. High-Resolution Transmission Electron Microscopy (HRTEM). HR-TEM has been employed to study the microtexture of the larger molecular-mass fractions of the NMP extracts from EC2386 coal, its partially gasified chars, and the bosh coke Core1. A 200KV field emission analytical transmission electron microscope (JEOL JEM 2010) instrument has been used to acquire the images. A few drops of the NMP extract solution were gently deposited onto carbon film grids (AGAR-S167-3 C/Cu, 300 mesh). The extractable materials were recovered by evaporation of the excess NMP in a vacuum at a temperature of about 120 °C. The dried grid was inserted into the microscope instrument to acquire the high-resolution images of these extracts.

Results and Discussion Apparent Molecular-Mass Distribution of the NMP (33) Li, C. Z.; Wu, F.; Cai, H.-Y.; Kandiyoti, R. UV-fluorescence spectroscopy of coal pyrolysis tars. Energy Fuels 1994, 8 (5), 1039–1048.

Extracts from Partially Gasified Chars. NMP extracts from EC2386 coal and its partially gasified chars were tested using SEC with a Mixed-A column. Their area-normalized chromatograms are shown in Figure 2. Chromatograms of the extract from the bosh coke Core1 and the WMR tars are also given for comparison. For clarity, only the 350 nm UV absorption lines have been shown. The chromatogram of the EC2386 coal extract consists of four peaks: a very low intensity peak spanning between 12 and 13 min, two peaks at between 13 and 16 min, and a broad and intense peak centered at ca. 22 min. The chromatogram of the NMP extract from the WMR char was noisy because of the low concentration of the extract solution. Nevertheless, peaks centered at ca. 16 and 23 min were clearly observed. The chromatogram of the WMR tars clearly showed a bimodal distribution, consisting of peaks centered at ca. 23 min and ca. 14.5 min. The chromatograms of the NMP extracts from two CGR chars were similar to that of the extract from the bosh coke Core1. A dominant peak, starting from 11 min and centered at ca. 12 min, was observed. The retained peak between 20 and 24 min showed relatively low intensity. At 20 min, the polystyreneequivalent mass is about 1200 units. Because the exclusion limit of the Mixed-A column occurs at ca. 14 min, peaks centered at ca. 12 min fall out of the linear range of the column calibration.15 As discussed in the Introduction, materials eluting from the column between 11 and 13 min are “soot-like”. In addition, the 12 min centered peaks are noted to be asymmetric, with the right tail extended to nearly 16 min. This suggests that the peak represents a broad distribution of molecular masses. Clear changes can be observed in Figure 2, regarding the apparent molecular-mass distribution of the NMP extracts from


samples exposed to environments representing different degrees of proneness to soot formation. The reference samples, i.e., the WMR char and tars, are considered, first. As a result of the configuration of the reactor, the WRM char is considered to be free of deposited secondary products. This is confirmed by the weak chromatogram of its NMP extract, which contains only materials of lower molecular mass. Moreover, the WMR tars do not elute from the SEC column before 13 min, with a considerable fraction retained (i.e., resolved) by the column (Figure 2). Therefore, the excluded materials, under the 11-13 min peak, extracted from CGR chars are likely to be retained secondary-reaction products derived from primary tars. Furthermore, the intensity of the retained peak (centered at ca. 22 min) of the NMP extracts from the CGR chars decreased significantly compared to that of the WMR tars. This indicates the destruction of primary tars (simulated as the WMR tars) during the combustion/gasification of the EC2386 coal in the CGR rig. It is worthwhile to compare the chromatograms of the extracts from the two CGR chars, derived from the same EC2386 coal, in some detail. The gasification conditions for preparing the two chars were nearly the same, except that the CGR181 char has seen a higher level of oxygen. As a result, its weight loss upon oxidation is higher than that of the CGR 179; its pyrolysis weight loss is slightly higher than that of the CGR 179, but the slight difference seems to be well within experimental error (see Table 2). In Figure 2, it is noted that the intensity ratio of the ca. 12 min peak to the retained peak (ca. 22 min) is lower for the CGR 181 extract than the CGR 179 extract. This difference implies that, in the more soot-prone environment, “primary” tars would crack to lighter materials to a greater extent. Meanwhile, some of the “primary” tars would also be thermally altered into very high molecular-mass materials. This trend can be extended to the bosh coke, Core1, which is considered to have had an even more sooting exposure in the blast furnace. This is consistent with the approximately constant sum of soot plus tars reported by other researchers.21-23 For Core1, the retained peak of its extract also shifts to longer elution time, suggesting that it contains even lighter materials than the two CGR chars. Figure 2, therefore, shows that secondary reactions of coal tars are closely associated with the formation of “soot-like” materials. Tars evolved during the rapid heating of coal are unlikely to burn as rapidly as small molecular-mass species. The often held view is that soot formation, in industrial-scale combustion/gasification processes, originates from the recombination of small molecules. In fuel-rich environments (or short residence times in oxidizing environments, as in a blast furnace), however, the sequence appears to be reversed, with tars probably simultaneously dehydrogenating and repolymerizing, to form “soot-like” materials. In fact, the explosive ejection of tars and tar precursors from pyrolyzing coal particles is well-known to produce cenospheres and trails of rapidly ejected tars;34-37 tar molecules have also been shown to form soot directly upon secondary pyrolysis.38 (34) Li, C. Z.; Bartle, K. D.; Kandiyoti, R. Vacuum pyrolysis of maceral concentrates in a wire-mesh reactor. Fuel 1993, 72, 1459–1468. (35) Gray, V. R. The role of explosive ejection in the pyrolysis of coal. Fuel 1998, 67 (9), 1298. (36) Qu, M.; Ishigaki, M.; Yokuda, M. Ignition and combustion of laserheated pulverized coal. Fuel 1996, 75 (10), 1155–1160. (37) Eatough, C. E.; Smoot, L. D. Devolatilization of large coal particles at hight pressure. Fuel 1996, 75 (13), 1601–1605. (38) Yu, L. E.; Hildemann, L. M.; Dadamio, J.; Niksa, S. Characterization of coal tar organics via gravity flow column chromatography. Fuel 1998, 77 (5), 437–445.

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Figure 3. Height-normalized synchronous spectra of NMP extracts from CGR chars, the WMR char and tars, EC2386 coal, and the bosh coke Core1.

A further understanding of the formation mechanism of “sootlike” materials requires that changes in the molecular structure be considered, because primary tars may undergo secondary reactions. UV fluorescence spectroscopy and FTIR spectroscopy have been used here to investigate the changes occurring. Molecular Structures of NMP Extracts from Partially Gasified Chars and Coke Surfaces. Observation of differences between UV fluorescence (UV F) spectra allows for qualitative structural comparisons to be made between complex coalderived materials. UV F spectroscopy, as applied to coal pyrolysis tars, has been discussed in detail elsewhere.33 Figure 3 shows the height-normalized UV F synchronous spectra of NMP extracts from CGR chars, the WMR char and tars, the EC2386 coal, and the bosh coke Core1, respectively. The peak at ca. 290 nm corresponds to the NMP solvent. The fluorescence signal for the extract from the WMR char is very weak and dominated by the NMP signal. This is expected, considering the low concentration of the extract solution. Moreover, signal intensity in the >450 nm region is low, suggesting the presence of only small amounts of large aromatic ring systems. The result appears consistent with the apparent molecular-mass distribution of the sample indicated by SEC (see Figure 2). The extract from the bosh coke Core1 showed a strong signal only at the shorter wavelengths, with a very weak signal in the region >450 nm. The strongest peak was not the NMP signal but the signal at about 310 nm from actual extractable materials. Although there was a substantial amount of “soot-like” materials in the extract, this extract does not give strong fluorescence signals. It has been shown in previous work39 that the materials excluded from the porosity of the SEC column do not fluoresce to any measurable extent. Coal and petroleum asphaltene-derived materials of masses greater than about 3000 units tend to have little or no fluorescence, because the quantum yields decrease sharply with an increase in the size of ring systems. Therefore, the observed fluorescence signal of the Core1 extract is from its fraction of low molecular mass rather than from the “sootlike” material. It is interesting to note that the maximum of the fluorescence intensity shifted to longer wavelengths in the following se(39) Morgan, T. J.; Millan, M.; Behrouzi, M.; Herod, A. A.; Kandiyoti, R. On the limitations of UV fluorescence spectroscopy in the detection of high-mass hydrocarbon molecules. Energy Fuels 2005, 19 (1), 164–169.



Figure 4. Height-normalized UV fluorescence spectra of the 20-25 min fraction of NMP extracts from the CGR chars, the WMR char and tars, and the EC2386 coal.

Figure 5. Height-normalized UV fluorescence spectra of the 13-16 min fraction of NMP extracts from the CGR chars, the WMR tar, and the EC2386 coal.

quence: the WMR tar < EC2386 coal extract < CGR181 extract < CGR179 extract. This shift clearly suggests that the concentration of larger aromatic clusters increases in the same sequence. The NMP extractable materials are considered to be formed as a result of thermal alteration of the tars evolved from coal during the combustion/gasification process.5 The thermal alteration hereby appears to make the tars more aromatic, with increasing concentrations of larger aromatic clusters. However, only a certain fraction of the whole extracts studied here, e.g., materials under the retained peak on SEC, gives significant UV fluorescence signal. It is necessary to know the upper limit of the UV F detectable mass range of these samples. Therefore, three fractions eluting from the SEC column at intervals of 11-13 min (designated as fraction 1), 13-16 min (designated as fraction 2), and 20-25 min (designated as fraction 3) were collected for analysis by UV F spectrscopy, respectively. Figure 4 shows the height-normalized UV fluorescence spectra of 20-25 min elution fraction from SEC (fraction 3) of the NMP extracts from the EC2386 coal, the CGR chars, and the WMR char and tars, respectively. The fluorescence spectrum of the WMR char extract was dominated by the NMP signal, with the peak centered at ca. 290 nm, suggesting that only small aromatic clusters are present in the WMR char extract. For the other four samples, shifting of the intensity maximum to longer wavelengths, implying increasing concentration of larger aromatic clusters, is found to follow the same sequence as for the whole extracts: the WMR tar < EC2386 coal extract < CGR181 extract < CGR179 extract. Because this 20-25 min fraction is of relatively low molecular mass, the shifting suggests that the change in molecular structures, upon secondary reactions of primary tars, occurs not only in the higher mass ranges as shown by SEC in Figure 2 but also within the lower mass fractions of evolving tars. Figure 5 shows the height-normalized UV F spectra of the 13-16 min fractions (fraction 2) of NMP extracts from the EC2386 coal, the CGR chars, and the WMR tars, respectively. The signals for all of the samples are quite weak, and the NMP signal dominates the spectra of all four samples. Clearly, a portion of this fraction is not able to give any fluorescence intensity at all. Three peaks, centered at 370, 420, and 530 nm, respectively, can be observed for the WMR tars. Among all of the samples studied, this fraction of the EC2386 coal extract gave the weakest intensity signal. Although the

absolute concentrations of the different samples were not kept exactly the same, they were comparable as a result of successive dilution with NMP. Therefore, the weakest fluorescence signal of the EC2386 coal fraction is due to its structural nature rather than low concentration. The local density of large aromatic ring systems appears to have been high for this fraction of coal extract. The UV F spectra of the 13-16 min fractions of the two CGR char extracts show maxima at ca. 480 nm and are similar. However, the intensity for the CGR179 char extract is slightly higher, near 480 nm. The 11-13 min fractions (fraction 1) of the NMP extracts from the EC2386 coal, the CGR chars, and the WMR char and tars were also analyzed using the UV F spectroscopy. Not surprisingly, only the signals of the NMP solvent were observed (not shown). SEC suggested this fraction was of high molecular mass and gave nearly no fluorescence (low quantum yields). Other techniques were therefore needed to characterize these fractions, which were of particular interest within this study. FTIR Spectroscopy of the Ultra Filtration Fractions (Nominally >100 000 Units). FTIR spectroscopy has been used to obtain information on the structural features of the 11-13 min fractions of extracts from EC2386 coal, CGR chars, and the bosh coke Core1. However, collection of enough sample from the analytical SEC column for FTIR analysis proved to be a very slow process. As an alternative, an ultrafiltration (UF) cell has been used to collect the larger molecular-mass fractions. The membrane used for the filtration had a separation cutoff of nominal molecular mass of 100 000 units (Millipore ultrafiltration membrane disk with NMWL g 100 000). The fractions retained on this membrane were recovered and washed by an acetone/water (4:1) mixed solvent to remove the NMP residue and then dried under vacuum overnight at 80 °C to remove the mixed solvent. The recovered nominally >100 000 unit fractions were retested on the same SEC system with NMP. Peaks spanning between 11 and 16 min dominated the chromatograms, with only very weak peaks between 22 and 25 min. Therefore, samples thus collected were thought to be representative of the fractions of larger molecular mass, which were undetectable to UV F spectroscopy. Figure 6 shows the smoothed FTIR spectra of the UF >100 000 unit fractions of NMP extracts from EC2386 coal, CGR chars, and the bosh coke Core1. Although the quality of the FTIR spectra is not good, peaks in the regions of interest can be discriminated from the baselines. The individual spectra


Dong et al.

this sample is very amorphous. Some concentric patterns are observed in the image, though blurred, of the CGR181 extract. Similar concentric patterns are also present in the TEM image of the CGR 179 extract. In addition, microtextures of graphene layers are found, marked by the white circle. It is arguable that similar graphene microtextures might also be present in the bulk UF >100 000 unit fraction of the CGR 181 extract but happened not to be detected in the portions viewed under TEM. Nevertheless, this finding, in combination with the FTIR spectroscopy results, increases our confidence that the UF >100 000 unit fraction of the CGR179 extract is more ordered than that of the CGR181 extract. Similar graphene layers are frequently found in the TEM image of the UF >100 000 unit fraction of the Core1 extract, as expected. This bosh coke has experienced high temperatures (ca. 1500 °C) for a long period of time in the highly soot-prone bosh region of the blast furnace. Figure 6. FTIR spectra of the nominally UF >100 000 unit fractions of NMP extracts from the CGR chars, the EC2386 coal, and the bosh coke Core1.

are arranged in sequence with the increasing degree of soot proneness, with EC 2386 representing the original coal. There is a decrease in the intensity of the (C-H)al stretching frequency (2800-3000 cm-1), under the more soot-prone conditions. This is consistent with more intense dehydrogenation as the environment becomes more soot-prone. Dehydrogenation should be accompanied by increased condensation of the aromatic ring system. Therefore, accompanying an increase in the intensity of the (C-C)ar peak would be expected. However, the quality of the peaks in the appropriate regions (100 000 Unit Fractions. TEM offers another independent technique for studying the structural changes of high molecularmass extracts from EC2386 coal, CGR chars, and the bosh coke Core1. The high-resolution TEM images of these samples are shown in Figure 7. No well-defined crystal-like microtextures are seen in the image of EC2386 coal extract, indicating that (40) Griesheimer, J.; Homann, K. H. In Proceeding of 27th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1998; p 1753. (41) Krestinin, A. V. In Proceeding of 7th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1998; p 1557. (42) Moss, J. B.; Stewart, C. D.; Syed, K. J. In Proceeding of 7th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1998; p 413. (43) Wornat, M. J.; Vernaglia, B. A.; Lafleur, A. L.; Plummer, E. F.; Taghizaden, K.; Nelson, P. F.; Li, C. Z.; Necular, A.; Scott, L. T. In Proceeding of 7th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1998; p 1677. (44) Mitchell, P.; Frenklach, M. In Proceeding of 7th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1998; p 1507.

Summary and Conclusions The present study aims to gain a better understanding of the formation mechanism of the “soot-like” materials, eluting between 11 and 13 min on SEC, found present in lower temperature regions of blast furnaces. The work has strived to improve the understanding of how these materials form, by tracing the changes in both molecular-mass distribution and molecular structures of the tar-derived NMP-extractable materials (NMP extracts), from samples that had experienced various degrees of propensity to form soot. Within this framework, the attachment of gas-phase species (mainly acetylene) to surfaces of soot nuclei particles has not been studied. The apparent molecular-mass distributions of these NMP extracts have been studied by SEC. The most important finding is that cracking of primary tars, simulated by the WMR tars, is accompanied by the build-up of “soot-like” materials. This indicates that the secondary reactions of coal tars are closely associated with the formation of “soot-like” materials in a blast furnace. Furthermore, the changes in the structural features of these extracts have been investigated in two steps. UV F spectroscopy has been used to study the structural changes in fractions appearing under the “resolved” (later eluting) peak of relatively low molecular mass (designated as “retained” materials). The UV F results suggested that the concentration of larger aromatic clusters increased in the sequence: WMR tars < EC2386 coal extract < CGR181 extract < CGR179 extract Extracts from the partially gasified CGR chars are believed to derive from the primary tars evolved upon heating. Therefore, a more soot-prone environment, e.g., in the case of the CGR 179 char, would appear to favor an increase in aromaticity of tar-derived extracts. FTIR spectroscopy and TEM have been used to characterize the higher molecular-mass fractions, which are primarily effluents between 11 and 16 min (i.e., the excluded peak in SEC) from the SEC effluent stream. This is because UV F spectroscopy is not able to provide structural information on the fractions of larger molecular mass (i.e., effluent eluting earlier than ca. 20 min from the SEC column). These high-molecular-mass fractions have been isolated using an ultrafiltration (UF) cell with a filtration membrane of nominal 100 000 unit cutoff. Despite the poor quality of the FTIR spectra, the results suggest that dehydrogenation occur more significantly when the coal has been gasified in a more fuel-rich environment. In parallel, the molecular structure seems to become more aromatic.



Figure 7. High-resolution TEM images of the UF >100 000 unit fractions of NMP extracts from (a) the EC2386 coal, (b) CGR181, (c) CGR179, and (d) the bosh coke.

Independently, TEM also shows that more graphene layers are present in the sample taken from a more soot-prone environment. When these observations are taken together, they indicate that primary tars are thermally altered to form more aromatic materials in a more soot-prone environment, eventually leading to soot formation. Thus, indicative structural information has been acquired across the apparent molecular-mass range for samples studied in this paper. An improved picture for the formation of “sootlike” materials has thus emerged. Tars, mixtures of a broad range of PAHs, are evolved from the injectant coal in the blast furnace. Tars would undergo secondary reactions, such as dehydrogenation and reploymerization/condensation, as

well as some cracking reactions, giving off light gases and other low-molecular-mass species. These secondary reactions would eventually lead to the formation of the “soot-like” materials of very large apparent molecular mass. Acknowledgment. The research project “Minimising Environmental Emissions by Optimised Reductant Utilisation” has been funded by the European Union under the Research Fund for Coal and Steel (Contract RFS-CT-2004-00004). The authors also thank Dr. C. Atkinson of Corus RD&T, Teesside Technology Centre, U.K., for technical advice and provision of samples. EF800466H

Investigating the Formation Mechanism of Soot-like Materials Present

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