Energy & Fuels 2003, 17, 565-570
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Size Exclusion Chromatography of Particulate Produced in Fuel-Rich Combustion of Different Fuels B. Apicella,* A. Ciajolo, R. Barbella, and A. Tregrossi Istituto di Ricerche sulla CombustionesCNR, P. le Tecchio, 80-80125 Napoli, Italy
T. J. Morgan, A. A. Herod, and R. Kandiyoti Department of Chemical Engineering, Imperial College, Prince Consort Road, London, SW7 2 BY, U.K. Received July 3, 2002
Carbonaceous particulates formed in the combustion of gaseous and liquid fuels were collected from a premixed laminar flame and a spray flame operating under fuel-rich combustion conditions. Particulate collected in the flame was extracted with dichloromethane (DCM) to separate condensed species (CS) soluble in DCM from solid carbonaceous material (soot), insoluble in DCM. These samples were examined by size exclusion chromatography (SEC) using N-methylpyrrolidone (NMP) as eluent, with detection by light scattering and UV-visible absorbance. Size exclusion chromatography of soot and related materials has provided a diagnostic tool for a more thorough characterization of these materials than has been hitherto available. The data show that there are step-changes in structures from the small near-planar PAH molecules, to large components with elution times corresponding to polystyrenes of mass above 200 000 u. The SEC analysis of soot, carried out on Mixed-A column with light scattering detection, showed an intense peak corresponding to molecular masses much larger than the highest mass polystyrene standard (15 × 106 u). The DCM-soluble fraction of the ethylene soot gave two peaks, the former corresponding to a molecular mass of the order of 1 × 106 u, whereas the second peak corresponded to masses ranging from several hundreds to about 200 u on the basis of polystyrene calibration; part of the material in this range was found to be polynuclear aromatic hydrocarbons (PAH) (128-300 u) as detected by gas chromatography-mass spectrometry. Using a column with a better resolution regarding the smaller molecules (Mixed-D) and connected to an UV-visible absorption detector, the molecular weights of soot and CS were found to be in good agreement with those obtained by using the previous SEC system. The molecular weight ranges of soot and CS were found independent of both the combustion system and fuel used even though some differences could be observed in the relative proportions of the different molecular masses.
Introduction Combustion systems commonly used for heating, transportation, energy production, etc., are the main sources of urban pollution due to the emission of organic and inorganic species having high toxicological effects on human health. Lung tissue irritation and respiratory diseases are known to result from the inhalation of soot particles, particularly the ultra fine particulate carbon,1 which is a typical product of fuel-rich combustion. Furthermore, chemical species such as polycyclic aromatic hydrocarbons (PAH) adsorb on soot and are known carcinogens.2,3 * Corresponding author. Fax:+39 081 593 6936. E-mail: apicella@ irc.na.cnr.it. (1) Dockery, D. W.; Pope, C. A., III. Annu. Rev. Public Health 1994, 15, 107-132. (2) Polynuclear Aromatic Hydrocarbons: Chemistry, Metabolism and Carcinogenesis, I; Frendenthal, R. I., Jones, P. W., Eds.; Raven: New York, 1976. (3) P.H. and Cancer; Gelboin, H. V., Ts’o, P. O. P., Eds.; Academic Press: New York, 1978.
To control the emission of PAH and particulate carbon from combustion processes it is necessary to know their formation mechanism. It is generally accepted that intact aromatic ring units such as benzene and polycyclic aromatic hydrocarbons, commonly detected in the soot inception region, play an important role in soot formation.4 However, the mechanism is far from being completely understood because high-molecular-weight species in the molecular mass range going from typical PAH detected by sampling and chromatographic analysis (108 u, as evaluated by mere extrapolation of the calibration curve. However, it is very hazardous to extrapolate the polystyrene curve to such high values that are probably above the exclusion limit and above the available largest mass calibration compound (15 × 106 u). The SEC chromatogram of a CS sample is also reported in Figure 2. It shows a bimodal distribution, with two chromatographic peaks at 14 and 21 min. The peak at 21 min mainly comprises lower-molecularweight aromatic material in the GC-MS range (up to 300-400 u). On the basis of the gas chromatographymass spectrometry analysis of the CS, it was found that PAH with molecular mass up to 300 u (named GC-MS PAH) constitute a fraction of total CS going from 30 up to 50 wt %. The presence of the peak at 14 min confirms that GC-MS PAH are only a part of total CS. The peak
at 14 min is located in the upper MW range and corresponds to a MW of about 2 × 106 u, much larger than that of the lower-molecular-weight aromatic material. This gap of MW is surprising also considering that a such heavy material should hardly dissolve in DCM. A structural change from the planar or near-planar structures of the material accessible to GC-MS analysis to a three-dimensional structure could cause the material to be eluted with smaller elution times (i.e., larger MW) giving an “apparent” larger molecular weight. This hypothesis should justify the anomalous behavior of a spherical molecule such as fullerene (720 u) that has been found to elute on a similar column at earlier times (i.e., larger MW) than those expected.13 Using the second system (Mixed-D column with UVabsorption detector) it is possible to have an enhanced detection at both higher and lower MW due to the highresolution power of the column and to the high sensitivity of the UV detector in regard to aromatic species. Consequently, more information about the MW of the samples and the presence of other masked species were expected with this kind of column. Nevertheless, the use of at least two different SEC systems is important for a reciprocal validation. Mixed-D chromatograms for diesel oil, rapeseed oil, and ethylene soot are reported in Figure 3. Three peaks at 6.8, 8.5, and 9.5 min can be noted indicating the higher resolution power and detection sensitivity of this SEC system with respect to the Mixed-A/scattering detector system. The first two peaks are above the exclusion limit of the column (about 10 min of elution time corresponding to a polystyrene MW of 200 000 u) and above the highest MW polystyrene standard (2 × 106 u). Moreover, the peak at 9.5 min indicates that soot contains also material in the 200 000-300 000 u range that is within the linearity range of the column (just in correspondence of exclusion limit). It is noteworthy that a separation of high-molecularweight soot species occurs well above the exclusion limit of the column as evaluated on the basis of polystyrene
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Figure 5. SEC chromatograms of soot and condensed species from ethylene flame on Mixed-D column (350 nm of absorption wavelength).
Figure 4. SEC chromatograms of condensed species from diesel oil, kerosene, ethylene (a) and from heavy fuel oil and rapeseed oil (b) on Mixed-D column (350 nm of absorption wavelength).
standards. This indicates that the structure and/or the conformation in NMP solution of very large MW species contained in soot samples is different from that of polystyrene standards. In other words the polystyrene calibration cannot be considered reliable for the evaluation of the MW of the largest soot particles. However, the structures of these very large molecules/particles remain to be discovered and further work will be necessary. The SEC chromatograms of heavy fuel oil soot and of the kerosene soot are not reported, since they were found to almost completely overlap the diesel oil soot and ethylene soot, respectively. This demonstrates that the molecular mass ranges of soot do not vary by changing both the combustion system (premixed and spray flame) and the fuel (ethylene, rapeseed oil, kerosene, diesel oil, and heavy fuel oil) and only a different distribution of single peaks can be observed (Figure 3). The Mixed-D SEC chromatograms of ethylene, kerosene and diesel oil CS (Figure 4a), and heavy fuel oil
and the rapeseed oil CS (Figure 4b) present two main regions with four peaks at 9, 10, 18, and 20 min confirming the higher resolution power and detection sensitivity of this SEC system. Also in the case of CS, the molecular mass ranges appear to be quite independent of the combustion system and the fuels. The 20 min peak is due to 200-300 u species and corresponds to the molecular weight of the GC-MS PAH. A shoulder on the PAH peak at 18 min is attributable to species with a MW of about 1000 u. Higher-molecularweight species of 2 × 106 u (peak at 9 min) and 200 000 u (peak at 10 min) are also present, confirming that there are many other high-molecular-weight species besides two- to seven-ring PAH. The CS material eluting in the upper polystyrene MW range (1 00 000-200 000 u) partially overlaps with the high-molecular-mass peak of soot, as shown in Figure 5 where the chromatograms of soot and CS on Mixed-D/UV-visible absorption detector are contrasted. However, as previously hypothesized in the case of soot, the high-molecular-mass peak of CS species could be due to species having a threedimensional structure rather than such a high MW. This could also justify the gap between high- and lowmolecular-weight species found in SEC analyses of CS that is in contrast with the general thinking of a progressive molecular weight growth in the soot formation mechanism.4 In other words, this gap could be due to the passage from the near-planar PAH structures to three-dimensional species and just this passage is the critical step in soot particle formation. More work is necessary to study the possible effect of structural changes, such as the passage from planar to spherical molecules, on SEC elution and hence on molecular weight determination. This implies also a search of standards alternative to classical polystyrenes. Further work is planned to give more insights into the structure of high-molecular-weight species contained in soot and CS in order to establish definitely a reliable molecular weight, size, and structure. Moreover, the approach will be extended to the analysis of soot and CS collected in the soot formation region of flames in
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order to follow the possible changes in the distribution of molecular weight as soot incepts and grows. Conclusions Size exclusion chromatography on polystyrene-divinyl benzene columns was performed by using NMP as eluent in order to evaluate the molecular weights of soot and condensed species collected in premixed and spray flames. Most of the soot species eluted in a sharp range of very high molecular weight (>2 × 106u) as evaluated on the basis of polystyrene standards. By contrast, the SEC chromatograms of condensed species presented a predominant peak in the range of 200-400 u belonging to polycyclic aromatic compounds, but other significant peaks in the elution range of high-molecular-weight material (about 200 000 u) were detected. The soot and condensed species chromatograms presented the same peaks independently of the fuel and of the combustion system even though some differences in the relative contribution of individual peaks was found. The gap between small polycyclic aromatic hydrocarbons molecules and very large molecular weight species, the latter present in traces in the condensed species and in massive amounts in soot samples, is in contrast with the general thinking of a progressive molecular growth
Apicella et al.
starting from PAH up to the first soot particles. It is possible that the evaluation of very high molecular weight species on the basis of polystyrene standards is not reliable for condensed species and soot samples. In the range of 200-500 u the evaluation of the molecular weight of polycyclic aromatic hydrocarbons on the basis of polystyrene standards was found to be correct; however, already a molecule with a relatively low molecular weight (720 u), but with a peculiar shape like that of fullerene, showed an apparent much higher molecular weight when evaluated by SEC separation using polystyrene standards. Moreover, it was found that the apparently high-molecular-weight species of soot and condensed species samples were still separated in a region where polystyrenes of MW larger than 200 000 u were not separated. Further work is necessary to establish if the large gap between small PAH and high-molecular-weight species is only apparent and could be trivially due to stepchanges in structure from near-planar PAH toward three-dimensional rigid structures not assimilable to polystyrene standards. The study of this effect is also crucial for understanding the still unknown mechanism of soot inception in flames. EF020149R
