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Investigating the Fate of Injectant Coal in Blast Furnaces by Size-Exclusion Chromatography S. Dong, L. Wu, N. Paterson,* A. A. Herod, D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom ReceiVed September 20, 2006. ReVised Manuscript ReceiVed NoVember 21, 2006
The fate of injectant coal in blast furnaces has been investigated through characterizing samples collected from commercial-scale blast furnaces and laboratory-scale tests. Representative samples, including an injectant coal and the corresponding carryover dusts from a full-sized blast furnace, and a set of tuyere level coredrilled samples from another full-sized blast furnace, have been examined. The samples were extracted using 1-methyl-2-pyrrolidinone (NMP) as the solvent, and the extracts were then studied using size-exclusion chromatography (SEC). The carryover dust extracts were found to contain heavy carbonaceous materials of apparent mass ∼107-108 units, on the basis of polystyrene calibration. Similar materials were also found to be present in the NMP extracts of the core-drilled samples taken from the bosh, rear of bird’s nest, and deadman regions of the furnace, at the tuyere level. In contrast, the feed coke did not give any extractable material. Controlled pyrolysis and combustion experiments, in an electrically heated wire-mesh reactor, suggest that the extent of injectant coal combustion in the raceway is limited by a very short exposure time to high temperatures and poor oxygen availability. These observations suggest that some coal char particles might escape from the raceway incompletely pyrolyzed. Unburned volatiles, particularly tars, may be further thermally altered, giving rise to the formation of high-molecular-weight (soot-like) materials.
Introduction Coal has traditionally been supplied in the form of metallurgical coke into blast furnaces. The practice of partially replacing increasingly expensive coke with injectant hydrocarbon (coal, oil, or natural gas) has become widespread. The 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 ∼200225 kg of coal/ton of hot metal, operational problems have been encountered. These include a substantial increase in the level of dust emission, a reduction in the permeability of the coke bed, an instability of the raceways, and poor drainage of molten slag and iron in the bosh zone2 (defined in Figure 1). Studying the fate of injectant coal in blast furnaces aims to improve the understanding of the furnace performance at high coal injection rates and to assist in the control of the blast-furnace operation. It is mostly convenient to sample carryover dusts and, occasionally, raceway core-drilled cokes, because of difficulties in obtaining other samples from furnaces running under high temperature and pressure conditions. The carryover dust samples are of particular interest, because these materials have been exposed to the full sequence of reaction conditions from the raceway to the top of the blast furnace. The core-drilled samples provide information on the reactions taking place between the bosh and deadman regions. Characterization of these samples is likely to provide clues about the fate of injectant coal. * To whom correspondence should be addressed. Telephone: +44-(0)20-7594-5634. Fax: +44-(0)20-7594-5638. E-mail: n.paterson@ imperial.ac.uk. (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, 716-724. (2) Bortz, S.; Flament, G. Experiments on pulverized coal combustion under conditions simulating a blast furnace environment. Ironmaking Steelmaking 1983, 10, 222-229.
Figure 1. Raceway core-drilling at the tuyere level in the blast furnace.
Most of the characterization work on coal-derived materials presented in this paper has been based on the extraction of samples. Suitable solvents for this purpose include dichloromethane, pyridine, and 1-methyl-2-pyrrolidinone (NMP). The extractable materials recovered from the samples may be characterized using a variety of analytical techniques. Molecularmass ranges that can be examined by gas chromatography (GC) or GC-mass spectrometry (MS) are limited to ∼300 units for aromatic compounds and ∼500 units for aliphatic compounds.3 Information on higher molecular-mass fractions needs to be obtained through the use of a different set of analytical methods. The present study has made extensive use of size-exclusion (3) Lazaro, M. J.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Calibration of size-exclusion chromatography in 1-methyl-2-pyrrolodinone for coalderived materials using standards and mass spectrometry. Energy Fuels 1999, 13, 1212-1222.
10.1021/ef060472k CCC: $37.00 © 2007 American Chemical Society Published on Web 01/06/2007
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Table 1. Coal and Carryover Dust Samples and Their Properties
sample BS2 BFb PJN-F PJN-4
chara (vol %, mmf) 2.0 3.0 2.4
cokea (vol %, mmf)
ash (% db)
VM (% db)
C (% db)
H (% db)
60.0 31.4 23.0
6.5 49.0 50.3 50.6
13.2 nd nd nd
85.9 nd nd nd
4.3 nd nd nd
a
The char and coke proportion were obtained by a point-counting technique at Corus, U.K. b Also contained 38% ferruginous mineral matterfree basis (mmf).
chromatography (SEC), which has been widely used to investigate the molecular-size distributions of organic species.4 In much of the early SEC work, tetrahydrofuran (THF) was used as the eluent5,6 but was found to have a number of limitations for coal-derived samples. These included the observed retention time dependency on structural features and the occurrence of adsorptive interactions between sample and packing, causing a delay in retention times. These problems were overcome by the use of a more powerful solvent, NMP, first used as an eluent in SEC by Lafleur and Nakagawa7 and developed further in the work performed at Imperial College London.3,4,8 In this study, a number of representative samples were extracted by NMP and examined using SEC, with NMP as an eluent. The SEC chromatographic features of the extractable materials in these samples were compared. The aim of the study has been to determine the origin of extractable materials in these samples, which reflects the fate of injectant coal in blast furnaces. Experimental Section Selection of Samples. A number of samples have been supplied by Corus Limited (U.K.), as follows: (i) A coherent set of samples from an operating blast furnace comprising the feed coke, the injectant coal (designated as BS2), together with three carryover dust samples (designated as BF, PJN-F, and PJN-4) have been studied. Properties of this set of samples are shown in Table 1. The char and coke proportions in the dust were determined using a Zeiss Universal reflected polarized light microscope. The differentiation between coke and char was based on overall morphology, particle shape, and degree of development of optical anisotropy, as revealed by interference contrast. It is evident from the data in Table 1 that not all of the injected coal is completely reacted, because a small portion of coal char is present in all three carryover dusts. In addition to studying the whole dusts, separate light and heavy fractions have been produced by density separation with chloroform. (ii) A complete set of core coke samples taken from an operating furnace, at the tuyere level, using the core-drilling technique9 shown in Figure 1, have been studied. (iii) A sample of coke recovered from the raceway region of a furnace rapidly (4) Apicella, B.; Ciajolo, A.; Suelves, I.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Structural characteristion of products from fuel rich combustion: An approach based on size-exclusion chromatography. Combust. Sci. Technol. 2002, 174, 345-359. (5) Bartle, K. D.; Taylor, N.; Mulligan, M. J.; Mills, D. G.; Gibson, C. Evaporative analyzer as a mass detector in the size-exclusion chromatography of coal extracts. Fuel 1983, 62, 1181-1185. (6) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Molecular mass calibration in size-exclusion chromatography of coal derivatives. Fuel 1984, 63, 1556-1560. (7) Lafleur, A. L.; Nakagawa, Y. Multimode size-exclusion chromatography with poly(divinylbenzene) columns and n-methylpyrrolidinone for the characterization of coal-derived mixtures. Fuel 1989, 68, 741-752. (8) 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. (9) Willmers, R. R. Coke properties in the bosh and raceway regions of the blast furnace. ReV. Metall./Cah. Inf. Tech. 1992, 89, 241-249.
Table 2. Experimental Conditions of the BS2 Char Preparation in the WMR conditions of sample preparation
sample
peak temp (PT) (°C)
heating rate (°C/s)
holding time at PT (s)
char1 char2 char3
700 1500 2000
1000 1000 1000
30 2 0
a
pressure (bara)
average total volatile (%, daf)a
standard deviation
1.5 3.0 3.0
11.0 18.5 19.3
0.23 0.85 0.75
daf ) dry ash-free basis. Table 3. Extractable Yields of Coal and Dust Samples extractable yield (% w/w)
samples coal BF dust
char coke
BS2 BF (1.5 h extraction) BF (6 h extraction) BF (re-extracted the residue) BS2 coal char1 feed coke frozen raceway coke
2.7, 3.2 6.6, 7.4 7.9 4.7
average (% w/w)
standard deviation
3.0 7.3
0.35 0.66
0 0 4.3
quenched with liquid nitrogen, at the end of its operating campaign, has also been studied. A second set of samples have been prepared in the laboratory from BS2 coal, using an electrically heated wire-mesh reactor (WMR); both tar and char samples have been generated. Details of the WMR system have been described elsewhere.10-14 The experimental conditions used and total volatile yields measured are shown in Table 2. Advantages of the WMR include accurate control of the time-temperature history of samples and suppression of the secondary deposition of evolving volatiles by passage of a stream of sweep gas past the heated sample. Finally, a candle-soot sample has been collected from the wall of a water-cooled tube inserted into a candle flame. Soot represents a suitable reference material for the highest molecular-weight (MW) pyrolysis products of coal volatiles. Extraction of Samples. NMP was used as the solvent because of its ability to achieve high extract yields under relatively mild conditions. In the present work, about 200 mg of sample was extracted in 10 mL of NMP for 1.5 h in an ultrasonic bath. The 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 extract yields, calculated by eq 1, are shown in Table 3. The repeatability of the extraction yields was (3%. percent extractable yield ) (weight of original sample - weight of residue) × 100% (1) weight of original sample (10) Kandiyoti, R.; Herod, A. A.; Bartle, K. D. Solid Fuels and HeaVy Hydrocarbon Liquids: Thermal Characterization and Analysis; Elsevier Science: Amsterdam, The Netherlands, 2006. (11) Cai, H. Y.; Kandiyoti, R. Effect of changing inertinite concentration on pyrolysis yields and char reactivities of two south African coals. Energy Fuels 1995, 9, 956-961. (12) Lim, J. Y.; Chatzakis, I. N.; Megaritis, A.; Cai, H. Y.; Dugwell, D. R.; Kandiyoti, R. Gasification and char combustion reactivities of Daw Mill coal in wire-mesh and “hot-rod” reactors. Fuel 1997, 76, 1327-1335. (13) Messenbo¨ch, R. C.; Dugwell, D. R.; Kandiyoti, R. Coal gasification in CO2 and steam: Development of a steam injection facility for highpressure wire-mesh reactors. Energy Fuels 1999, 13, 122-129. (14) Gu¨ell A. V.; Kandiyoti, R. Development of a gas-sweep facility for the direct capture of pyrolysis tars in a variable heating rate high-pressure wire-mesh reactor. Energy Fuels 1990, 7, 943-952.
1064 Energy & Fuels, Vol. 21, No. 2, 2007 SEC. SEC on a mixed-A column (7.5 mm in diameter × 300 mm in length) from Polymer Laboratories (U.K.) has been used in the present work. The column was packed with polystyrenepolydivinylbenzene beads of 20 µm in nominal diameter. NMP was used as the eluent because it has been shown to elute molecules by a predominantly size-exclusion mechanism and with minimum surface effects in contrast to other solvents such as pyridine or THF,7,15,16 where surface interaction effects are more important. 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 bar; 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, polymethylmethacrylate (PMMA) up to ∼1 × 106 units, and polysaccharides (PSAC) up to 788 000 units.8,17 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 with the polystyrene calibration. Structural features of these compounds ranged from polynuclear aromatics to dyes and organic acids.8 (b) Mass determinations up to m/z 3000, by matrix-assisted laser desorption ionization (MALDI)-MS, of fractions of pitch from SEC8,18 were comparable with the equivalent polystyrene 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), poly(ethylene-adipate) (10 000 units), and poly(vinylpyrrolidone) (3500 units),8,17eluted close to the polystyrene calibration curve.
Results and Discussion Extraction Results. As shown in Table 3, no material could be extracted from the feed coke sample or the BS2 coal char1 sample prepared in the WMR (heating at 1000 °C s-1 to 700 °C and then held for 30 s). This is expected, because metallurgical coke is treated in a high-temperature environment for many hours; the BS2 coal was held in the WMR for 30 s at 700 °C. As a check, the NMP used for extraction of BS2 char-1 was run through the SEC; no peaks appeared in the chromatogram, confirming the absence of extractable materials. However, extracts of the other two chars prepared at higher peak temperatures but at a near-zero hold time at the peak temperature gave intense peaks in their SEC chromatograms, although their total volatile yields are greater. The following sequence of events is envisaged. The NMP extraction removes material in an unaltered form, which is trapped within the macromolecular structure of the coal. The total volatile yield is determined by measuring the amount of material removed during the thermal breakdown of the coal structure under test conditions; the peak temperature is the dominant effect here. As the temperature increases progressively, large coal molecules break down to small molecules, some of which may contribute toward increas(15) Johnson, B. R.; Bartle, K. D.; Herod, A. A.; Kandiyoti, R. N-Methyl2-pyrrolidinone as a mobile phase in the size-exclusion chromatography of coal derivatives. J. Chromatogr., A 1997, 758, 65-74. (16) 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: 1-Methyl-2-pyrrolidinone as mobile phase in size-exclustion chromatography. Energy Fuels 1996, 10, 743-750. (17) 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, 335-361. (18) 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, 1813-1823.
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ing the volatile yield. However, the amount of pyrolytic materials trapped in the complex coal/char matrix structure may also increase. It is these materials that are capable of being extracted out of the char matrix by NMP as the “solubles”. However, the trapped materials rapidly repolymerise to form insoluble char if exposed to temperatures of 700 °C and above, if exposed for more than several seconds.19 The total absence of NMP extractables in the feed coke implies that any extractable carbonaceous material identified in the blast-furnace carryover dust can only have originated from the injected coal. Similarly, under real furnace conditions, any char particles reaching the top of the furnace must have spent many seconds in successive regions of the furnace, where, initially, temperatures far exceed 700 °C. The chars in blastfurnace dust would not be expected to carry extractable material. Any NMP-extractable material found in blast-furnace dust is thought to arise from surviving tarry material, originally volatilized during passage through the tuyeres and raceways, the hottest zones of the blast furnace. In contrast to the absence of extractable material in feed coke, the coke sample recovered from the frozen raceway was found (Table 3) to contain approximately 4% of material extractable by NMP. This would appear to reflect the deposition onto the raceway coke of tarry materials evolving from the injectant coal in the tuyeres and raceway zones. Table 3 also shows that increasing the extraction time to 6 h did not increase the extractable yield significantly above the 1.5 h extraction level. However, about 5% more extractable could be washed out when re-extracting the already extracted BF dust with fresh NMP. The extractable yields of the carryover dusts are based on the initial weight of the dusts, which are mixtures of carbonaceous materials, i.e., char, coke, and extractables, together with ferruginous materials, fluxes, and refractory lining fragments. The variable quantity of these noncarbonaceous components means that dust extract yields cannot be compared directly with each other or with that from the feed coal, BS2. Nevertheless, the observed extractable yields, of around 7%, are unexpectedly high for dusts that have experienced the high temperatures common within a commercial blast furnace. It is therefore interesting to characterize these soluble materials with the aim of providing some insight into the complex reactions occurring within the blast furnace. SEC Chromatograms of Extracts. Figure 2 shows SEC chromatograms of NMP extracts from the BS2 coal sample; four separate peaks are observable. The peak at ∼22 min mainly comprises lower MW material, probably in the GC-MS range (up to 300-400 units). At 20 min, the polystyrene equivalent mass would be about 1200 units. The peak at 13-14 min is located in the upper MW range and corresponds to a MW of about 2 × 106 units according to the polystyrene calibration curve. Peaks eluting before 14 min represent very heavy molecules (or particles), and the peak at about 12 min corresponds to a polystyrene calibration standard MW in excess of 107 units. However, these very high values should be regarded only as indications, because they are based on the extrapolation of the linear region of the polystyrene calibration curve. These early peaks elute before the exclusion limit of the column and before the largest available polystyrene standard (15.5 × 106 units). Clearly, the nature of these materials is not well-defined. An alternative, three-dimensional, calibration for SEC has been made using soot, colloidal silicas, and fullerene (19) Fukuda, K.; Dugwell, D. R.; Herod, A. A.; Kandiyoti, R. Effect of rapid preheating on the coking behaviour of bituminous coals: Retrogressive reactions and extract yields. Energy Fuels 2004, 18, 1140-1148.
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Figure 2. SEC chromatogram of the BS2 coal NMP extract.
Figure 3. SEC chromatogram of the NMP extract from the BF dust light fraction.
as standards; here, a sphere with a diameter of 10 nm would elute at about 12 min.8 It is similarly possible that the early eluting components from BS2 coal may be three-dimensional molecules (or particles) of undetermined mass, which are unable to penetrate the column porosity because of their shape. Increased noise was found to be present in the SEC chromatogram of NMP extracts of the BF dust because of the dilute nature of the sample solution. The NMP extract of the BF dust light fraction, prepared by density separation of the whole dust using chloroform (Figure 3), gave a similar profile to that of the BS2 coal NMP extract (Figure 2). The short elution time peaks were absent in chromatograms of whole blast-furnace dust extracts. This shows the importance of the density fractionation of the BF dust. The light fraction, 1% by weight of the whole dust, is much less in quantity than the heavy fraction, so that the early peaks are probably masked in the whole dust by the heavy fraction. In conclusion, the highest molecular-mass extractable material is only present in detectable quantities in the light fraction. It is too dilute for identification in the NMP solution of the whole BF dust or in the heavy fraction. Figure 3 shows that the earliest peak of the BF dust light fraction extract appears at about 11-11.5 min, which represents molecules even larger than the heaviest molecules observed in BS2 coal. However, in a previous study of soot, Apicella et
al.20 found that the fraction of ethylene flame soot insoluble in dichloromethane eluted as a sharp, single peak centered at ∼11 min on the same mixed-A column. A chromatogram of candle soot has been prepared as part of the present study to verify this observation; a peak at around 11 min was again observed (Figure 4). Primary soot particles from fuel-rich combustion systems have been examined in several other studies.21-28 Evidence from other analytical techniques applied to soot formation in flames (20) 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, 345-359. (21) Hessler, J. P.; Seifert, S.; Winans, R. E. Small-angle X-ray scattering for studies of soot inception and formation. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 2002, 47, 736-737. (22) Choi, K.; Mochida, I. Characterization of particulate matter emitted from diesel engine. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 2002, 47, 776-777. (23) Blevins, L. G.; Yang, N. Y. C.; Mulholland, G. W.; Davis, R. W.; Steel, E. B. Early soot from inverse diffusion flames. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 2002, 47, 740-741. (24) Glasier, G. F.; Pacey, P. D. Observation and collection of an aerosol in ethane at 1184 K. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 2002, 47, 742-743. (25) 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, 4021-4026.
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Figure 4. SEC chromatogram of the candle soot NMP extract.
Figure 5. SEC chromatogram of the NMP solution of the BS2 coal tar1 sample prepared in an atmospheric pressure WMR.
indicates that diameters of primary soot particles range from 20 to 60 nm.21-24 Soot with a diameter of about 40-50 nm has been detected in the dust from the blast-furnace operation where coal injection was used.28 Soot particles from ethylene combustion were reported to have diameters of about 60 nm,25 of similar size to those of the other flames, while sizes of primary soot particles from diesel engines range between 18 and 33 nm.26 Apicella et al.27 have confirmed these observations on a soot sample, examining fractions of the soot passed or stopped by a 20 nm filter. The >20 nm fraction of soot was excluded from the column porosity, as in observations described above. It seems reasonable to propose that the sizes of material (particles) observed under the earliest peak of the BF dust light fraction correspond closely to soot. (26) 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, 1887-1892. (27) Apicella, B.; Barbella, R.; Ciajolo, A.; Tregrossi, A. Comparative analysis of the structure of carbon materials relevant in combustion. Chemosphere 2003, 51, 1063-1069. (28) 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.
In Figure 3, the peaks at ∼13-14.5 min correspond to a molecular mass of about 2 × 106 units, on the basis of linear polymer calibration, which is similar to the mass estimated for the heavy fraction of the NMP extracts of the condensed species from an ethylene flame.20 Peaks at similar retention times were observed in the chromatogram of the candle-soot sample and BS2 tar1 sample (see Figure 5), with a very low intensity in the latter. The peaks at about 22.5 min (Figure 3) are observed for all of the samples, which mainly comprise lower MW aromatic material reaching up to about 1000 units. Extracts of two more blast-furnace carryover dust samples were similarly examined by SEC to see if the soot-like material of very high apparent molecular mass was present. These dusts were collected from the same blast furnace but with an operation at lower coal injection rates. Light fractions of these two samples were prepared by the density separation method using chloroform. However, in neither sample was any peak observed to elute before ∼14 min (SEC chromatogram of one of the samples is shown in Figure 6). The excluded peak in this sample is particularly large because the baseline does not return to zero before the 23 min peak starts. These observations suggest that soot-like material is not always present in carryover dusts.
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Energy & Fuels, Vol. 21, No. 2, 2007 1067
Figure 6. SEC chromatogram of the NMP extract from PJN-4 blast-furnace carryover dust.
Figure 7. SEC chromatogram of the NMP extract of BS2 char2 prepared in the WMR.
It is likely that the presence of soot is observed only under certain coal injection conditions. Certainly, high coal injection rates would cause more volatiles and tars to be released which, in turn, would be expected to generate more of the soot-like materials found in the carryover dust. It is also interesting that the chromatographic features of extracts from the PJN-4 (Figure 6) and PJN-F (not shown) dusts were similar to those of the BS2 coal tar1, although the intensity ratio of large to small molecular-mass peaks is much higher in the PJN dusts. The BS2 coal tar1 sample was prepared under conditions minimizing the effect of secondary reactions. In contrast, the materials extracted from the dust light fraction have encountered the sequence of temperatures zones within the blast furnace, highest in the tuyeres and raceway and progressively lower as the sample moves up through the gasification zone. The hightemperature exposure would be expected to cause secondary reactions, including cracking, repolymerization, and dehydrogenation, leading mostly to light volatiles, but also a small fraction of high MW material, observed by the higher intensity ratio of large to small molecular-mass peaks in the SEC chromatograms. As mentioned previously, no material could be extracted from the BS2 char1 sample prepared by rapid heating in the WMR
(holding for 30 s at 700 °C). Exposure times longer than about 5 s at 700 °C effectively tend to allow the remaining tar precursors to bake onto the char.19 However, the SEC chromatograms of the BS2 char2 and char3 samples (Figures 7 and 8, respectively) show clear peaks centered at ∼16.5 and ∼24 min, indicating that devolatilization of these two chars was not completed under the conditions of the preparation in the WMR (1500 °C, 2 s hold; and 2000 °C, 0 s hold, respectively). It is important to note that it is these short residence time conditions rather than those used in the preparation of BS2 char1 that most closely resemble the conditions experienced by injectant coal in the raceway and the front of the bird’s nest region. The presence of extractables in particles exposed to such high temperatures suggests that not all of the coal mass actually sees the external temperature in the very short residence times allowed. Parallel experiments have been carried out, using the highpressure version of the WMR, to simulate combustion conditions experienced by injectant coal particles during their passage through the tuyere and raceway. The extent of combustion observed under conditions simulating those in the tuyere and raceway is surprisingly low (Figure 9). In these experiments, conducted at 3 bara, the layer of coal particles, held within the folded wire mesh, was heated to the
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Figure 8. SEC chromatogram of the NMP extract of BS2 char3 prepared in the WMR.
Figure 9. Extent of combustion of coal at different peak temperatures in the high-pressure WMR (3% O2 in the purge gas, 105 ms residence time in the mesh, under 3 bara).
peak temperature under a nitrogen gas sweep. As the peak temperature was reached, the sweep gas flow was switched to one containing 3% O2. After just 105 ms, the sweep gas flow was changed back to nitrogen. The average extent of extra weight loss, directly attributable to combustion, observed over and above the normal pyrolysis volatile loss, was less than 3.5%, even at 1600 °C. It should be noted that the coal residence time in a real blast-furnace tuyere and raceway is even shorter, normally about 25 ms. These experiments provide solid evidence that conditions inside the tuyere and raceway do not favor the complete combustion or even complete pyrolysis of all of the injectant coal. The observations of incomplete coal combustion in the WMR simulation may be explained by considering the likely fate of coal particles injected into a real furnace. Coal particles are injected into the blowpipe immediately upstream of the tuyere, giving rise to the virtually instantaneous release of a plume of volatiles constrained within the blowpipe. This particle-laden plume then accelerates through the tuyere into the furnace, reaching velocities perhaps half that of the local velocity of sound. Meanwhile, particles on the periphery of the plume pyrolyze and combust first. Volatiles released by these particles are more likely to consume the oxygen in the blast air preferentially, thereby depriving particles within the plume of the oxygen necessary to affect combustion. Gas concentration
measurements on a working furnace indicate that the oxygen concentration drops to zero close to the tuyere nose.29 Surviving coal char particles cross the raceway in less than 20 ms. They may be recycled within the raceway, become embedded within the adjacent coke mass, or pass toward the upper regions of the furnace where the temperature drops progressively. Because of the short times involved, the particles passing into and out of the raceways may still be able to release some volatiles. The nearly oxygen-free and fuel-rich atmosphere represents an ideal soot-forming environment for tar components in the mixture. Repolymerization and dehydrogenation reactions would be expected to take place, leading to the formation of very high molecular-mass materials, alongside cracking reactions that serve to break up most of the tars. Evidence for this sequence of events is provided by the SEC chromatograms of NMP extracts from coke samples recovered by core-drilling an operating furnace at the tuyere level (see Figure 1). The analysis of core-drilled coke samples is particularly significant, given the radial variation in temperature, coke bed porosity, and coke residence time across the furnace, from the bosh to deadman regions.9 The SEC chromatograms of extracts of the coke samples taken from the raceway (Figure 10) and in the front of the bird’s nest are similar, containing little other than weak peaks from very small molecules centered at ∼15 and ∼23 min. However, peaks centered at ∼12 min, with the forward edge at 11 min, are seen in the chromatograms of the bosh coke (Figure 11), the coke from the rear of the bird’s nest, and the deadman coke. These three coke extracts have similar chromatograms dominated by the excluded peaks at ∼12 min but with very small peaks in the retained region, centered at ∼23 min. A comparison of these chromatograms with those of the BF dust light fraction (Figure 3) and BS2 tar1 (Figure 5) suggests that the excluded peaks are high-molecular-mass secondary reaction products, which display characteristics similar to soot (Figure 4). The presence of high-molecular-mass materials in these core-drilled coke samples is consistent with the presence of 4% NMP extractables found in a coke sample recovered from (another) frozen raceway supplied by Corus (deliberately quenched with liquid nitrogen at the end of the furnace-operating campaign). It is apparent that the time-temperature history of the coal particles/volatiles inside the blast furnace is paramount in (29) Ishii, K. AdVanced PulVerized Coal Injection Technology and Blast Furnace Operation; Elsevier Science Ltd.: Oxford, U.K., 2000; pp 6869.
Injectant Coal in Blast Furnaces by SEC
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Figure 10. SEC chromatogram of the raceway coke sample.
Figure 11. SEC chromatogram of the bosh coke sample.
determining the extent of formation of soot-like materials from the injectant coal tar. However, the detailed mechanisms of formation of these high MW materials are not yet understood. Summary and Conclusions The aim of the present work is to study the fate of injectant coal in blast furnaces. Carryover dust samples and core-drilled samples from operating furnaces have been extracted by NMP as the solvent and characterized using SEC. The chromatographic features of these samples have been compared to those produced from char extracts and tar samples prepared in the laboratory, using an electrically heated WMR designed to simulate conditions in the actual furnace. The study has shown that core-drilled samples recovered from an operating furnace at the tuyere level and also the carryover dust samples swept out of the furnace contain materials that are soluble in NMP. The absence of such NMP-extractable material in the fresh feed coke suggests that the extractables found in the blast-furnace samples must be derived from the injectant coal. Furthermore, the soot-like nature of the heaviest extractable material suggests that it must be derived from tars ejected from the injectant coal and then polymerized and dehydrated to soot. Their observance in the very hot zones of
the furnace (at the level of bosh and deadman) suggests that these species are chemically stable. In contrast, coal samples rapidly (1000 °C s-1) heated to 700 °C and held for 30 s gave no extracts. When these findings are taken together, they suggest that soot formation occurs from ejected tars in oxygen-deficient regions prior to their adherence to coke particles. In the furnace simulation experiments conducted in the WMR, it was found that the exposure time at peak temperature rather than the peak temperature itself determined the extractable yield of the residual char. Thus, the injectant coal was not completely pyrolyzed even at 2000 °C, with a 0 s hold time, or at 1500 °C, with a 2 s hold time. Experiments in the high-pressure WMR show that the extent of combustion of the pyrolyzed char is small (less than 5%) in the short time available (20-30 ms) in the raceway. These observations imply that the injectant coal is not completely pyrolyzed and certainly not completely combusted in the raceways of blast furnaces. Thus, part of the residual char from the injectant coal, still containing some volatiles, may escape from the raceway into cooler regions of the furnace. Continuing volatile release may then lead to the formation of heavy soot-like materials by a secondary reaction. The details of such reactions are not yet clearly understood. In conclusion, a better understanding of the fate of injectant coal inside blast furnaces has been developed. Further work is
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now underway to improve the understanding of the nature of secondary reactions of volatile matter released from the injected coal. Acknowledgment. The research project, “Minimising Environmental Emissions by Optimised Reductant Utilisation”, has been
Dong et al. funded by the European Union under the Research Fund for Coal and Steel (contract number RFS-CT-2004-00004). The authors also thank Dr. C. Atkinson of Corus Limited (U.K.) for technical advice and provision of samples. EF060472K
