68
Energy & Fuels 2004, 18, 68-76
The Fate of Volatiles from Injectant Coals and Soot Formation in Blast Furnaces Suneerat Pipatmanomai, Alan A. Herod, Trevor J. Morgan, Nigel Paterson,* Denis R. Dugwell, and Rafael Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College London, SW7 2AZ, U.K. Received April 24, 2003. Revised Manuscript Received September 29, 2003
The relative abundance of coke particles, injectant coal-char, and soot found in blast furnace dust provides clues about the operation and efficiency of the blast furnace. The aim of the work has been to determine whether the source of the soots could be related to the injectant coal, in particular, to the nature of the extractable material within it. This paper describes the separation of soot from blast furnace dust and its characterization. Dust samples collected during coal injection tests in a blast furnace and in a pilot scale, “single-tuyere” test rig have been extracted with NMP (1-methyl-2-pyrrolidinone). The parent coals used in these tests were also extracted with NMP; the solvent is highly polar and useful in dissolving various types of soot as well as coal extracts and tars. Fresh coke gave no extract. The coal and dust extracts were characterized by size exclusion chromatography (SEC). Differences were observed between chromatograms of the extracts from the two coals usedsthe first in the “single-tuyere” test rig, and the second in the blast furnace. SEC was useful in identifying similar features in the chromatograms of extracts from the parent coal and the soot extracted from the blast furnace dust; similar observations were made on the “single-tuyere” test rig dust and the coal injected into it. These findings support the view that tar evolution from the injectant coal contributes directly to soot formation and is a major contributor to levels of dust emission observed at high coal injection rates.
Introduction Pulverized coal is injected into blast furnaces to provide thermal energy and to conserve coke. However, during operations with high rates of coal injection, soot formation1 is thought to cause reductions in coke bed porosity and high dust emission rates. Dust exiting the top of a blast furnace with the gases is comprised of ferruginous material, abraded coke particles, fine injectant coal char particles, and soot. The gas itself contains high concentrations of CO, but must be dedusted prior to use as fuel. A fuller understanding of dust formation processes would help in identifying ways of suppressing levels in the exit gas, as well as limiting carbon losses from the furnace. It is often thought that the formation of acetylene and low mass dienes provides the initial building blocks in soot formation. However, there is evidence2 showing that soot and VOC (volatile organic compounds) formation in flue gases may originate with pathways other than recombinations of small molecules and radicals.3 In particular, where injectant coal devolatilization involves large proportions of tar release, direct dehydrogenation appears to be the more likely route for forming soot particles. The explosive ejection of tars and * Corresponding author. E-mail:
[email protected]. (1) Bortz, S.; Flament, G. Conference on direct use of coal in iron and steelmaking 1982, 10, 1, 222. (2) Herod, A. A.; Lazaro, M.-J.; Suelves, I.; Dubau, C.; Richaud, R.; Shearman, J.; Card, J.; Jones, A. R.; Domin, M.; Kandiyoti, R. Energy Fuels 2000, 14, 1009-1020. (3) Shufen, L.; Ruizheng, S. Fuel 1994, 73 (3), 413.
tar precursors from rapidly pyrolyzing coal particles has been experimentally observed to produce cenospheres (char) and trails of rapidly ejected tar, which has been exposed to very high temperatures.4-7 These dehydrated tar molecules have SEC signatures similar to those of ordinary soot and appear to form directly upon secondary pyrolysis.8 However, detailed mechanisms of soot formation in blast furnaces are not completely understood. Blast furnace soot is much less reactive than residual coal char,9 and once past the tuyeres and raceways, its complete consumption in the blast furnace would require higher air/fuel ratios and more reactive environments than is commonly practiced in blast furnace operation. In the examination of blast furnace dust, part of the emission is ascribed to small particles abraded from the coke bed itself. Such emissions may be enhanced by increases in gas velocity through the bed, caused by the high rates of volatiles release from injectant coal particles. On the other hand, coke itself is not expected to release volatiles (as confirmed below) and soot formation is not thought likely to originate with the coke bed itself. Distinguishing coke dust from injectant coal-char (4) Gray, V. R. Fuel 1988, 67, 1298. (5) Qu, M.; Ishigaki, M.; Yokuda, M. Fuel 1996, 75, 1155. (6) Eatough, C. E.; Smoot, L. D. Fuel 1996, 75, 1601. (7) Li, C.-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 1459-1468. (8) Yu, L. E.; Hildemann, L. M.; DaDamio, J.; Niksa, S. Fuel 1998, 77, 437. (9) Smoot, L. D.; Pratt, D. T. Pulverised coal combustion and gasification; Plenum Press: New York, London, 1979; p 149.
10.1021/ef030101t CCC: $27.50 © 2004 American Chemical Society Published on Web 12/04/2003
Fate of Volatiles from Coals and Soot in Blast Furnaces
Energy & Fuels, Vol. 18, No. 1, 2004 69
Figure 1. “Single-tuyere” rig (Teesside Technology Centre).
particles would also have been useful and is being investigated by numerous researchers. The distinct morphologies of char and soot particles have been observed by TEM (transmission electron microscopy)10,11 and SEM (scanning electron microscopy)12,13 in a sample from a pilot-scale furnace. Soot is observed as spherical, carbon-rich, submicron-sized particles, whereas char particles appear as porous and carbon-rich particles. An attempt to quantify soot and char yields was carried out by Veranth et al.,13 by means of liquid-suspension gravity separation. Tars and soots may be separated from feeds, bed materials, and collected dusts by extraction with suitable solvents, such as dichloromethane or pyridine.14-17 These materials are characterized using a variety of analytical techniques. Molecular mass ranges that can be examined by GC or GC-MS is limited to ∼300 µm for aromatic compounds and ∼500 µm for aliphatic compounds.18 Information on higher molecular mass fractions is therefore obtained through the use of different analytical methods. The present study has made extensive use of size exclusion chromatography (SEC), following the observation that soots from different sources could apparently be dissolved in NMP (1methyl-2-pyrrolidinone). SEC has been widely used to investigate the molecular size distribution of organic species. It is able to detect large molecular mass fractions in materials, including soot.19 In much of the early SEC work, tetrahydrofuran (THF) was used as the eluent,20,21 but was found to have a number of limitations. These included the observed retention time dependency on structural features, adsorptive interactions between sample and packing causing a delay in the elution times of the samples, and partial precipitation of heavy materials. These problems were overcome by the use of the more powerful solvent, NMP, first used as an SEC eluent by Lafleur and Nakagawa22 and developed further in the current work.2,18,19 In this study, extractions in NMP (as solvent) and size exclusion chromatography with NMP (as eluent) have been used to isolate soots from process dust and to compare chromatographic features of soots and extracts from the same coal. The aim of the work has been to determine whether the source of the soots could be
related to the injectant coal and to the nature of the extractable material within it. Experimental Section Samples. The study was carried out using two coals. The first (designated as BS2) had been used in blast furnace operations at Port Talbot and the second (designated as S/B3) in a “single-tuyere” test rig at the Teesside Technology Centre, Corus (UK) Ltd. The corresponding dust samples were (i) the carryover dust from the blast furnace (sample code: BF), and (ii) three dust samples (sample code: T1, T2, and CY) collected from the “single-tuyere” test rig. The test rig and the sampling positions are shown in Figure 1.23 T1 and T2 were collected from conditioning towers 1 and 2, respectively. Dust CY was collected from the cyclone, which collects the dust remaining in the exhaust gas at the exit from the two conditioning towers. A study on the reactivity of the coals using a wire mesh reactor under conditions that simulate the raceway of a blast furnace has already been reported.24 Two metallurgical coke samples were also examined. The first was a sample of “clean” coke taken from the feed stream (10) Gao, Y.-M.; Shim, H.-S.; Hurt, R. H.; Suuberg, E. M. Energy Fuels 1996, 11, 457. (11) Veranth, J. M.; Pershing, D. W.; Sarofim, A. F.; Shield, J. E. 27th Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, 1998. (12) Mclean, W. M.; Samuelson, G. S.; Heap, M. P.; Pohl, J. H. 18th Symposium (Int.) on Combustion; The Combustion Institute: Pittsburgh, 1980; p 1239. (13) Veranth, J. M.; Fletcher, T. H.; Pershing, D. W.; Sarofim, A. F. Fuel 2000, 79, 1067. (14) Chen, J. C.; Castagnoli, C.; Niksa, S. Energy Fuels 1992, 6, 264. (15) Fletcher, T. H.; Solum, M. S.; Grant, D. M.; Critchfield, S.; Pugmire, R. J. 23rd Symposium (Int.) on Combustion; The Combustion Institute, Pittsburgh, 1990; p 1231. (16) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1987, 1, 431. (17) Mclean, W. M.; Samuelson, G. S.; Heap, M. P.; Pohl, J. H. 18th Symposium (Int.) on Combustion; The Combustion Institute, Pittsburgh, 1980; p 1239. (18) Lazaro, M. J.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 1999, 13, 1212. (19) Apicella, B.; Ciajolo, A.; Suelves, I.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Proceedings of Second Mediterranean Symposium on Combustion, Egypt, January, 2002. (20) Bartle, K. D.; Taylor, N.; Mulligan, M. J.; Mills, D. G.; Gibson, C. Fuel 1983, 62, 1181. (21) Bartle, K. D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Fuel 1984, 63, 1556. (22) Lafleur, A. L.; Nakagawa, Y. Fuel 1989, 68, 741. (23) Willmers, R. R.; Fellows, P. M. “Effect of blast furnace coal injection upon bosh coke properties, coke combustion and furnace permeability”, Reduction of ores, British Steel Corporation, Final report, 1988. (24) Pipatmanomai, S.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2003, 17, 489-497.
70
Energy & Fuels, Vol. 18, No. 1, 2004
Pipatmanomai et al.
Table 1. Samples and Their Properties sample
ash (%db)
V.M. (%db)
C (%db)
H (%db)
BS2 S/B3 T1 T2 CY BFa
6.5 8.7 23.9 26.5 40.9 49.0
13.2 17.4 0.5 0.1 0.2 n.d.
85.9 82.7 80.2 77.1 63.8 n.d.
4.3 4.3 0.04 0.06 0.14 n.d.
a
Note that the BF dust also contains approximately 38 vol % ferruginous materials. Table 2. Extractable Yields of Coal and Dust Samples samples
extractable yield (% w/w)a
standard deviation
coal
S/B 3 BS2
12.2 17.1
2.8 1.2
dust
T1 T2 CY BF
8.7 9.5 9.8 17.7
1.4 0.3 0.8 1.4
0.0 4.3
2.8
coke
feed coke raceway coke
a
Data presented were obtained from the results of at least two extractions.
to the blast furnace and the second was a sample of coke picked from a frozen raceway of an actual blast furnace, displayed at the Teesside Technology Centre, Corus (UK) Ltd. Elemental analyses of the samples are given in Table 1. Extraction of the Samples. Solvent extraction can be used to remove components of coal without breaking covalent bonds. Up to 66% of a bituminous coal25 was soluble in NMP at 202 °C (boiling temperature of NMP) after refluxing for 1 h. In this work, NMP was used as the solvent due to its ability to achieve high extractable yields under relatively mild conditions. About 200 mg of sample was extracted in 10 mL of NMP, using an ultrasonic bath (1.5 h at room temperature). The solution was then passed through a 1 µm filter paper. The filter cake was removed from the filter paper and washed with a 4:1 acetone/water mixture in an ultrasonic bath for another 0.5 h, to remove the remaining NMP. The solution was filtered again and the solid residue dried overnight, in a vacuum oven at 80 °C. The extractable yield was calculated from
% extractable yield ) (weight of original sample - weight of the residue) × 100% weight of original sample The extractable yields of the coal, coke, and dust samples are shown in Table 2. The repeatability of the extraction yield was within (3%. Analytical Methods. Size Exclusion Chromatography. Instrument features have been described elsewhere.26 In this work, a column packed with 5 µm size polystyrene/polydivinylbenzene beads (“Mixed-D”; Polymer Laboratories Ltd., UK), was used for the majority of the work. NMP was used as the eluent in SEC, as it has been shown to give a size exclusion mechanism with minimum surface effects and better molecular mass information than other solvents, such as pyridine or THF.22,26,27 The SEC column conditions for the “Mixed-D” column were the following: temperature 80 °C, inlet pressure 80-120 bar, flow 0.5 mL min-1. Detection was by UVabsorption detection at 280, 300, 350, 370, and 450 nm. A limited amount of work on ethylene-derived soot has been done (25) Zondlo, J.; Stiller, A.; Stansberry, P.; Irwin, C. Proc. Pittsburgh Coal Conf. 1988, A12, 58. (26) Johnson, B. R.; Bartle, K. D.; Herod, A. A.; Kandiyoti, R. J. Chromatogr., A 1997, 65, 758. (27) Herod, A. A.; Zhang, S. F.; Johnson, B. R.; Bartle, K. B.; Kandiyoti, R. Energy Fuels 1996, 10, 743.
Table 3. Proportions of Coal and Coke Derived Material in the Dust Samples sample
char, vol %, mmf
coke, vol %, mmf
T1 T2 CY BF a
79 51 94 2
21 49 6 60
a Also contained 38% ferruginous material mmf: mineral matter free basis.
with a “Mixed-A” column, which has a larger pore size and can be used to characterize larger material than the “MixedD” column. This operates at room temperature, with an inlet pressure of approximately 20 bar.
Results and Discussion Preliminary Characterization of the Coal and Dust Samples. An initial characterization of the dusts was conducted in the Laboratories at Corus (UK) Ltd. They were examined using an in-house method using an optical microscope, which was capable of discriminating between char (i.e., coal-derived) and coke (i.e., derived from the coke bed in the equipment and the coke feed). The results are shown in Table 3. This shows that a large proportion of the fines released from the “singletuyere” rig was coal-derived. The blast furnace dust also contained coal-derived char, but at a much lower concentration. This is explained by the much higher coke inventory in the blast furnace and the presence of other materials, such as ore and limestone flux. The significance of the data shown on the Table 3, is that it shows that char is not completely reacted and can be emitted as fines from the equipment. These data do not give an indication of the tar/soot content of the fines. The tar content of the exit gases was not measured directly in either the blast furnace or “single-tuyere” rig, as it was considered that, because of the extreme conditions in both reactors, any tars released in the tuyere and raceway regions would be completely degraded to soot-type material by the time the gases reached the exit. However, an estimate of the tar released in the tuyere/raceway region can be made from studies done using a wire mesh reactor.28 In this work it was found that approximately 8% (by weight) of the BS2 coal was released as a primary tar. This figure must represent the maximum amount of tar that would be formed during the initial degradation of the volatile matter in the coal. It is this material that produces the soot as it experiences the high temperatures within the raceway and furnace bed. Some soot will be trapped within the coke bed itself, the remainder will be present in the fines either as discrete particles or trapped within the fine coke and char. We have not attempted a quantification of the total amount of soot formed from the coal volatiles, as not all of it will have been emitted in the fines. However, we do present information below on the proportion of the fines that are soluble in NMP solvent, it is this material that is classed as soot. Extractable Yields. Dust samples from the blast furnace and the “single-tuyere” test rig were extracted using NMP, as were the samples of coal and coke (Table 2). (28) Pipatmanomai, S. Ph.D. Thesis, University of London, 2002.
Fate of Volatiles from Coals and Soot in Blast Furnaces
The extractable yields of S/B 3 and BS2 coals were 12.2 and 17.1%, respectively. The volatile matter contents of the two coals were 17.4 and 13.2% for the S/B3 and BS2 coals, respectively. The different trends exhibited by the two sets of data are explained as follows. The NMP extraction removes material in an unaltered form, which may have been trapped within the macromolecular structure of the coal. The standard volatile matter content is determined by heating the coal in an inert atmosphere to 900 °C and measuring the weight loss. It therefore measures the amount of material that can be removed by thermal breakdown of the coal structure under the test conditions. The two procedures are therefore measuring different properties of the coal. The observation that the coals exhibit varying differences between the results obtained with the two procedures indicates that there is a measurable difference in the nature of the two coals tested and this may be reflected in the nature of the extractable material. No material could be extracted by NMP from the fresh sample of metallurgical coke. This was expected, as the sample had been “coked” in a high-temperature environment for many hours. This simple finding implies that any extractable material identified in the blast furnace fines could not have originated from volatiles in the coke charge. By contrast, Table 2 shows that the sample of coke taken from the frozen raceway was found to contain approximately 4% of material extractable by NMP. This would appear to reflect the deposition of material originating in the volatiles from the injectant coal in that particular blast furnace. Table 2 also shows that the extractable yields were between 8 and 10% for the “single-tuyere” rig dusts (T1, T2, and CY) and 17.7% for the dust collected from the commercial blast furnace (BF). Since the extractable yields of the dusts are based on the initial weight of carryover dusts, which are mixtures of carryover materials and includes char, coke, and some ferruginous materials (in the case of BF dust), they cannot be directly compared with the extractable yields of their corresponding coals. Nevertheless, the observed extractable yields of dusts are unexpectedly high for particles that have experienced the high temperatures within the blast furnace. Coal-derived material exposed to the very high temperatures within the raceway and base of the coke bed would not be expected to contain soluble material other than soot. Size Exclusion Chromatography of Soots and Coal Extracts. The calibrations of the SEC columns used in this study are based on (i) model compounds up to slightly above 1000 µm and with structural features ranging from polynuclear aromatics to dyes and organic acids; 29 (ii) a continuous array of polystyrene standards up to 15.5 × 106 µm and poly(methyl methacrylate) standards up to ∼1 × 106 µm;29,30 (iii) mass determinations by MALDI-MS of fractions from size exclusion chromatography up to 3000 µm;31 (iv) several polymeric standards of known molecular (29) Lazaro, M. J.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Energy Fuels 1999, 13, 1212-1222. (30) Herod, A. A.; Zhuo, Y.; Kandiyoti, R. J. Biochem. Biophys. Methods 2003, 1687, 1-27. (31) Islas, C. A.; Suelves, I.; Herod, A. A.; Kandiyoti, R. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 2002, 47 (2), 638-639.
Energy & Fuels, Vol. 18, No. 1, 2004 71
Figure 2. Normalized SEC chromatograms of samples run on “Mixed-D” column, at 350 nm UV absorbance: candle soot (curve 1), coal soot (curve 2), wood soot (curve 3), and naphthalene pitch (curve 4).
mass: poly-N-vinylcarbazole (90 000), polyvinyl acetate (170 000), poly(vinylpyrrolidone) (58 000), polyethyleneadipate (10 000), poly(vinylpyrrolidone) (3500).30 SEC studies by Lazaro et al.,18 using NMP as eluent, have also shown that fractions excluded from the porosity of the SEC column and emerging at early elution times were likely to be large molecular mass materials, rather than aggregates of small polar molecules. Working with petroleum asphaltene solutions in nitrobenzene, and also in pyridine, Sheu et al.32 have observed discontinuities in surface tension measurements as a function of concentration. The effect has been ascribed to the agglomeration of asphaltene micelles. These inferences were based on an analogy with observations on surfactants; nevertheless, the conclusions appear reasonable. However, the concentrations at which Sheu et al. observed discontinuities in surface tension measurements [log(concentration in weight %) ) -1.5] are about 3-4 orders of magnitude greater than sample concentrations estimated to be present in the SEC-detectors. On the other hand, as will be shown below, soot molecular masses determined on the basis of existing calibrations appear very large indeed and contain material (with sizes > 20 nm), which may actually be separated out by microfiltration. While it does not seem possible to ascribe actual molecular masses to these species, it is convenient in the present context to think of the sizes of soot particles in terms of elution times or apparent molecular masses, particularly for purposes of internal comparison. Figure 2 presents SEC chromatograms of NMP extracts of several soots, showing peaks at shorter retention times (∼6 min) than the “exclusion limit” of the “Mixed-D” column. According to the polystyrene calibration, the exclusion limit of this column at 9-10 min corresponds to ∼200 000-400 000 µm. The shorter elution time of ∼6 min appears to correspond to the void volume of the column2 and suggests that the largest molecules were carried through the column at or near the speed of the solvent front. The transmission electron microscope patterns of these soots suggest that the samples had amorphous, covalent carbon structures, rather than layered or (32) Sheu, E. Y.; De Tar, M. M.; Storm, D. A. Surface activity and dynamics of asphaltenes in asphaltene particles in fossil fuel exploration, recovery, refining and production processes; Sharma, M. K., Yen, T. F., Eds.; Plenum Press: New York & London, 1994; p 118.
72
Energy & Fuels, Vol. 18, No. 1, 2004
Figure 3. Normalized SEC chromatogram of filtered soot samples, > and < 20 nm fractions, run on “Mixed-D” column, at 350 nm UV absorbance.
fullerene-type structures.2 The same study reported that the sizes of these heavy materials could be of the order of 20 nm, as indicated by microfiltration and transmission electron microscope (TEM). This agreed with later SEC work by Apicella et al.19 Figure 3 shows the >20 nm fraction of an ethylene flame soot (separated by micro filtration), eluting between 6 and 7 min. According to the calibration of the “Mixed-A” column (of greater porosity than the “Mixed-D” column), the apparent molecular masses of these particles were greater than 107-108 µm as estimated by light scattering.19,33 A similar early eluting peak was observed for soots produced by fuel-rich combustion of a wide range of fuels (rape seed oil, petroleum fractions, etc.), prepared using a variety of combustion systems.19 SEC of Extracts of Coal Samples Injected into the Commercial Scale Blast Furnace and the “Single-Tuyere” Test Rig. SEC chromatograms of NMP-extracts of the untreated BS2 and S/B 3 coals are presented in Figures 4 and 5, respectively. The UVdetectors were set to determine absorption at five different wavelengths. Both extract samples absorb at all wavelengths, indicating that they are complex mixtures. Detection at a series of wavelengths provides partial, but useful, information about structural differences between eluting extract molecules, since larger PCA groups absorb more intensely at longer wavelengths. The extract from BS2 coal (Figure 4) showed two peaks, the first at elution times between 10 and 12 min and the second between 15 and 20 min, together with a very minor peak at 9-10 min. The extract from S/B 3 coal (Figure 5) showed two peaks, at elution times between 8 and 11 min and between15 and 20 min. The earlier peaks, i.e., those centered at about 10 min, represent the material excluded from column porosity, with apparent molecular masses above 200 000 µm. The peaks at longer elution times, i.e., those at about 1520 min, show signals from smaller molecular mass materials (from several hundred to somewhat above 6000 µm), according to the column calibration. Although the two extracts showed peaks at nearly similar elution times, some structural differences may be discerned. The difference in relative peak heights in the two diagrams indicates a difference in the ratio of (33) Islas C. A. Ph.D. Thesis, University of London, 2001.
Pipatmanomai et al.
large to small molecular mass materials, between the extracts from the two coals. A comparison between Figures 4 and 5 shows that the S/B3 coal extract contained a higher ratio of large to small molecular mass materials. Furthermore, when (height of early peak/height of late peak) ratios are compared for each wavelength, it is observed that the ratio gets bigger with increasing detector wavelength. As larger PCA groups absorb more intensely at longer wavelengths, these findings suggest that the sizes of PCA groups may be increasing with increasing molecular size. These findings are consistent with earlier observations made by a combination of SEC and UV-fluorescence spectrometry.34 The SEC chromatogram of the extract from S/B3 coal, produced under reflux in boiling NMP for 1 h, is shown in Figure 6. Comparison with Figure 5 shows that increasing the extraction temperature increases the ability of the solvent to extract a greater proportion of large molecules (i.e., the height of the peak centered at 10 min has increased relative to the peak at 18 min). However, this does not change the elution times of the peaks or the order of the wavelengths. This is in line with the previous study35 investigating the behavior of the extracts from Daw Mill coal and a biomass at different extraction temperatures. Larsen and Mohammadi36 have explained the increase in extractable yields by increased release of material held in the coal structure by weaker noncovalent interactions: van der Waals forces, London dispersive forces, and π-π interactions. SEC of Extracts of Dusts Recovered from the Commercial-Scale Blast Furnace and the “SingleTuyere” Test Rig. SEC chromatograms of the extracts of the dust samples are shown in Figure 7 (BF dust) and in Figures 8-10 (“single-tuyere” test rig dusts). A higher noise level, compared to those of the extracts of the coals, was observed for all of the dust extracts. This was due to the lower concentration of extracted soot in the solvent. The figures show the detector output as a function of time for a single wavelength only, as this improves the clarity of the plots. The signals at the other detector wavelengths showed a similar pattern. Sample concentration by evaporation was attempted, but did not improve the intensity of the signals. The extract in the evaporated solutions was found to precipitate, rather than remain in solution. The extract from the BF dust (Figure 7; from BS2 coal) gave peaks at elution times similar to those of the extract from BS2 coal (compare with Figure 4). Overall, the distribution was found to have shifted toward slightly earlier retention times, i.e., to greater apparent molecular sizes. A small peak was also observed at a shorter elution time (at about 6.5 min), consistent with soot formation in the blast furnace. Soot formed during the combustion of ethylene (Figure 3) only showed signals at the “void” volume (∼6.5 min) and the exclusion limit (∼9 min) of the column. This material is expected to have been formed through (34) Li, C.-Z.; Wu, F.; Cai, H.-Y.; Kandiyoti, R. Energy Fuels 1994, 8, 1039. (35) Richaud, R.; Lazaro, M.-J.; Lachas, H.; Miller, B. B.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 2000, 14, 317. (36) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107.
Fate of Volatiles from Coals and Soot in Blast Furnaces
Energy & Fuels, Vol. 18, No. 1, 2004 73
Figure 4. SEC chromatograms of the extract from BS2 coal at room temperature.
Figure 5. SEC chromatograms of the extract from S/B 3 coal at room temperature.
Figure 6. SEC chromatograms of the extract from S/B 3 coal under reflux at 202 °C under nitrogen atmosphere.
well-known reaction pathways originating with acetylene and low molecular mass dienes. By contrast, similarities between the SECs of the extracts from the coal and dust recovered from the blast furnace (Figures 4 and 7) clearly show the presence of similarly structured material in the two samples. Particularly noticeable is the signal from low molecular mass material in the 15-20 min regions of the two chromatograms. The
long residence times (at least minutes, at temperatures above 700 °C) of injectant coal chars in the blast furnace do not allow us to imagine that any extractables could have survived within the particles. It appears reasonable to infer from these data that at least part of the soot formed in the blast furnace was produced by a mechanism other than that of complete destruction to one- or two-carbon species, prior to the re-construction
74
Energy & Fuels, Vol. 18, No. 1, 2004
Pipatmanomai et al.
Figure 7. SEC chromatograms of the extract of BF dust sample at room temperature.
Figure 8. SEC chromatograms of the extract from T1 dust sample at room temperature.
of sootlike structures. Rather, our observations favor the view that at least part of the “soot” extracted from the blast furnace dust had originated in the carbonization/ dehydrogenation of tarry material released by the injectant coal. Chromatograms of “single-tuyere” test rig dusts also showed signals at three distinct elution times (Figures 8-10). In all three diagrams, a shift of the molecular mass distribution to earlier retention times (i.e., larger molecular size) may be observed; furthermore, the peaks at longer elution times (smaller molecules), between 15 and 20 min, were found to be less intense, compared to Figure 5, and to have shifted to 19-21 min. The low intensity of the low mass signal, compared to Figure 5, may in part be explained by the lesser presence of lower mass material in the original S/B3 coal, as observed earlier. However, it is likely that the depletion of low mass material was mostly caused by the time-temperature regime experienced by these particles; the latter is not known to any degree of precision, but appear to have been somewhat more intense in the “single-tuyere” test rig compared to the actual blast furnace. The extracts from all dust samples showed the presence of soot: peaks at about 6.5 min were observed quite distinctly in the chromatograms of all three samples derived from the “single-tuyere” test rig. These peaks were not observed in extracts of untreated coals (e.g., Figures 4 and 5), while the soot samples (Figures 2 and 3) all showed similar peaks at elution times of about 6 min. These peaks are thought to correspond to colloidal
particles, with elution times shorter than a 1.84 million µm polystyrene standard.2 The chromatogram of the T1 dust sample (Figure 8) showed a peak centered at about 10.5 min, nearly coincident with the excluded peak in Figure 5, for the extract of S/B3 coal. Together with the low mass signal at around 21 min, the data indicate that the T1 dust sample contains some of the material coming directly from the injectant coal. T2 and the CY cyclone dusts showed progressively weaker evidence to the same effect, the latter possibly because smaller (cyclone captured) particles would have experienced the intense reaction conditions more uniformly. The Fresh Coke and the Raceway Coke Sample. The unused blast furnace feed coke was found to yield negligible extract. This was expected, as potentially extractable material would have either been released and removed from the oven during the early stage of coke manufacture, or incorporated into the coke structure during the fluid stage of the process. This suggests that the large molecular mass material, observed in the carryover dusts, must have originated from the injectant coal. In a test conducted by Corus (UK) Ltd, the raceway of a blast furnace was frozen after a period of coal injection. Samples of the coke from the region of the raceway were recovered for analysis. A small proportion of this coke (∼4%) could be extracted using NMP. Since the fresh coke itself had been shown to yield negligible extract, it could be inferred that the material dissolved in NMP must also have originated from the injectant
Fate of Volatiles from Coals and Soot in Blast Furnaces
Energy & Fuels, Vol. 18, No. 1, 2004 75
Figure 9. SEC chromatograms of the extract of T2 dust sample at room temperature.
Figure 10. SEC chromatograms of the extract of CY dust sample at room temperature.
Figure 11. SEC chromatograms of the extract of the raceway coke sample at room temperature.
coal and had been trapped by the coke. Due to the dilute nature of the sample, the extract from the raceway coke sample gave weak UV-absorbance signal with a high level of noise (Figure 11). However, two peaks could be observed, centered around 10 and 18 min, suggesting that the material probably originated in coal-derived tar deposited within the coke structure. This can occur within the blast furnace as the ascending gases flow through the pore structure of the coke bed. During its passage, pyrolysis tars crack or dehydrogenate and deposit on the coke surfaces.
Summary and Conclusions Samples of two injectant coals and their dusts recovered from a blast furnace and a “single-tuyere” test rig have been extracted in NMP, to isolate soots from process dust. These extracts have been examined by size exclusion chromatography, using NMP as eluent, to compare chromatographic features of soots and extracts from the same coal. The aim of the work has been to determine whether the source of the soots could be related to the injectant coal and to the nature of the extractable material within it.
76
Energy & Fuels, Vol. 18, No. 1, 2004
This study has shown that the coke within the blast furnace and dust carried out of the furnace contain fractions that are soluble in NMP solvent. The inability to find extractable material in fresh coke (i.e., before it is fed to the furnace) suggests that the extractable material in the blast furnace samples must have been derived from the coal injected into the raceway. Similarities have been observed by size exclusion chromatography, between the extracts of the initial coals and the extracts of the carry-over dusts, strongly suggesting that the coal must be the source of the extractable material in the dusts. This implies that a proportion of the injected coal is escaping through the coke bed as soot. This may be as separate particles or as material trapped within the coke fines that are also carried out of the furnace. The formation and persistence of these products of the cracking of the coal volatiles gives rise
Pipatmanomai et al.
to some of the operating problems that have been encountered during coal injection: these include pore blockage in the bed and smoke emission. These problems get worse as the injection rate is increasedsa condition that will increase the extent of formation of the primary tar that leads to soot formation. Acknowledgment. The authors thank the European Union for funding this project under ECSC Contract No. 7220-PR-070. The authors also express their gratitude to Drs. Ron Willmers and Colin Atkinson of Corus (UK) Ltd. for providing the samples and for their unstinting support during the inception and execution of this work. EF030101T
