Depletion of Fuel Aromatic Components and Formation of Aromatic

In-flame sampling followed by chemical and spectroscopic analysis coupled with laser-induced fluorescence (LIF) measurements excited in the UV region ...
0 downloads 0 Views 153KB Size
Download PDF
Energy & Fuels 2001, 15, 987-995

987

Depletion of Fuel Aromatic Components and Formation of Aromatic Species in a Spray Flame as Characterized by Fluorescence Spectroscopy A. Ciajolo,*,† B. Apicella,‡ R. Barbella,† A. Tregrossi,† F. Beretta,† and C. Allouis‡ Istituto di Ricerche sulla Combustione, C.N.R., P. le Tecchio 80, 80125 Napoli, Italy, and Dipartimento di Ingegneria Chimica, Universita` di Napoli “Federico II”, P. le Tecchio 80, 80125 Napoli, Italy Received February 27, 2001

In-flame sampling followed by chemical and spectroscopic analysis coupled with laser-induced fluorescence (LIF) measurements excited in the UV region (266 nm) were performed on a spray combustion system burning a diesel oil and a vegetable aromatic-free fuel like a rapeseed oil. A large fluorescence emission peaked in the UV was found in the spray region of the diesel oil flame attributed to the unburned aromatic species of the fuel. In the rapeseed oil flame, the absence of aromatic species in the fuel justify the lack of the fluorescence emission in the UV region. The comparison of LIF spectra measured in the PAH-formation region with the fluorescence spectra of the high molecular weight species sampled both in the diesel oil flame and in the rapeseed oil flame suggests a relationship between the broad and unstructured fluorescence measured in the visible and these high molecular weight species formed in the pyrolytic regions of spray flames. The visible emission feature could be assigned to flame-formed PAH species contained in the high molecular weight species if their fluorescence spectra are shifted toward the visible for effect of the high temperature flame environment. Alternatively, responsible for visible emission could be the unidentified heavier part of the high molecular weight species that are know to fluoresce mainly in the visible range of the emission spectrum.

Introduction The most part of practical combustion systems commonly used for energy production, transport, and domestic heating are based on the combustion of spray of liquid fossil fuels such as diesel oils and heavy fuel oils. The environmental impact of this kind of combustion processes is related to the large emission of organic pollutants such as soot and polycyclic aromatic hydrocarbons (PAH). This is due to both the heterogeneity and the turbulence of the spray combustion process which locally creates fuel-rich regions where pyrolytic processes favor soot and PAH formation. Moreover, some peculiar characteristics of diesel oils and heavy fuel oils such as the low volatility and high aromatic content are known to largely affect soot and PAH formation1 and, consequently, final emission in the atmosphere. To control the formation and the consequent emission of these pollutants it is important to characterize the spray combustion process, but the turbulent and heterogeneous nature of this process requires the use of suitable diagnostic tools. The high temporal and spatial * Author to whom correspondence should be addressed: Dr. Anna Ciajolo. Tel./Fax: +39 0817682252/ +39 081 5936936. E-mail: [email protected]. † Istituto di Ricerche sulla Combustione, C.N.R. ‡ Universita ` di Napoli “Federico II”. (1) Haynes, B. S. Fossil Fuel Combustion; Bartok, W., Sarofim, A. F., Eds.; John Wiley & Sons: New York, 1991; p 261.

resolution and nonintrusive nature of optical methods render these methods particularly suitable for characterizing the spray flame structure in real time identifying the flame regions and the conditions where organic pollutants are preferentially formed.2,3 Laser light scattering (LLS), laser-induced incandescence (LII), and extinction measurements are the optical techniques able to determine the particle diameter, number density, and volume fraction of soot particles and, in spray combustion, these techniques have been used to study both the atomization process and the formation of solid particulate like soot and cenospheres typically formed in spray combustion.2 Laser-induced fluorescence (LIF), early measured in fuel-rich premixed flames by using visible light sources,2-6 is considered the optical technique more suitable for PAH and, in general, for aromatic pollutant detection. The visible broad band fluorescence has been (2) D’Alessio, A. Particulate Carbon: Formation During Combustion; Siegla, D. C., Smith, G. W., Eds.; Plenum Press: New York, 1981; pp 207-256. (3) Eckbreth, A. C.; Bonczyk, P. A.; Verdieck, J. F. Prog. Energy Combust. Sci. 1979, 5, 253-322. (4) Haynes, B. S.; Jander, H.; Wagner, H. Gg. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 585-592. (5) D’Alessio, A.; Di Lorenzo, A.; Borghese, A.; Beretta, F.; Masi, S. Proceedings of the Combustion Institute; The Combustion Institute: Pittsburgh, PA, 1988; Vol. 16, pp 695-708. (6) Muller-Dethlefs, K. Optical studies of soot formation and the addition of organic peroxides to flames. Ph.D. Thesis Dissertation, Imperial College, London, 1979.

10.1021/ef0100451 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/05/2001

988

Energy & Fuels, Vol. 15, No. 4, 2001

tentatively ascribed to molecular fluorescence of PAH by considering that they are the flame-formed species having a large quantum efficiency, much higher that the other pyrolysis molecules, like polyenes and poliynes and mono-ring aromatic compounds also present in fuelrich combustion conditions. However, the very low absorption cross section and fluorescence emission of PAH in the visible region, as measured by spectrofluorimetry in solution,7-9 is against the hypothesis of the predominant role of PAH as responsible of visible fluorescence. Nevertheless, it has been suggested that the high-temperature flame environment should affect the spectroscopic properties of PAH causing the fluorescence shift toward the visible, and some studies on the behavior of PAH injected in flames appeared to confirm this hypothesis.10,11 Further experimental studies have been performed by using both laser and lamp sources12-18 on diffusion flames, where the spatial location of fluorescence is quite well discriminated with respect to the soot spatial location as inferred by scattering measurements. A good correlation between fluorescence intensity and PAH concentration measured in a diffusion flame has been found16 corroborating the attribution of fluorescence to PAH. The recent availability of laser sources powerful in the UV region has allowed the possibility of extending the emission detection region in the UV where PAH significantly absorb and fluoresce.7-9 UV-excited LIF has been used to detect radical species such as OH and CH in diffusion flames and a background fluorescence, extending from UV into the visible, was found interfering on radical fluorescence and simply attributed to PAH.13,19-21 UV-excited LIF measurements devoted to follow aromatic structures formation in relation to their probable role in soot inception have been also performed in premixed22 and diffusion flames.12,23 (7) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Academic Press: New York, 1971. (8) Karcher, W.; Fordham, R. J.; Dubois, J. J.; Glaude, P. G. J. M.; Lighart, J. A. M. Spectral Atlas of Polycyclic Aromatic Compounds, Vol. 1; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1983. (9) Karcher, W. Spectral Atlas of Polycyclic Aromatic Compounds, Vol. 2; Kluwer Academic Publishers: Norwell, MA, 1988. (10) Coe, D. S.; Steinfeld, J. I. Chem. Phys. Lett. 1980, 76, 485489. (11) Alden, M. Ph.D. Thesis Dissertation, Lund Institute of Technology, Lund, Sweden, 1982. (12) Smyth, K. C.; Miller, J. H.; Dorfman, R. C.; Mallard, W. G.; Santoro, R. J. Combust. Flame 1985, 62, 157-181. (13) Smyth, K. C.; Shaddix, C. R.; Everest, D. A. Combust. Flame 1997, 111, 185-207. (14) Miller, J. H.; Mallard, W. G.; Smith, K. Combust. Flame 1982, 47, 205-214. (15) Gomez, A.; Littman, M. G.; Glassman, I. Combust. Flame 1987, 70, 225-241. (16) Prado, G.; Lee, M. L.; Hites, R. A.; Hoult, D. P.; Howard, J. B. Proceedings of the Combustion Institute: The Combustion Institute: Pittsburgh, PA, 1977; Vol. 17, pp 649-661. (17) Beretta, F.; Cincotti, V.; D’Alessio, A.; Menna, P. Combust. Flame 1985, 61, 211-218. (18) Petarca, L.; Marconi, F. Combust. Flame 1989, 78, 308-325. (19) Puri, R.; Moser, M.; Santoro, R. J.; Smyth, K. C. Proceedings of the Combustion Institute; The Combustion Institute: Pittsburgh, PA, 1992; Vol. 24, pp 1015-1022. (20) McEnally, C. S.; Pfefferle, L. D. Combust. Flame 2000, 121, 575-592. (21) Bohm, H.; Kohse-Hoinghaus, K.; Lacas, F.; Rolon, C.; Darabiha, N.; Candel, S. Combust. Flame 2001, 124, 127-136. (22) D’Alessio, A.; D’Anna, A.; D’Orsi, A.; Minutolo, P.; Barbella, R.; Ciajolo, A. Proceedings of the Combustion Institute; The Combustion Institute: Pittsburgh, PA, 1992; Vol. 24, pp 973-980.

Ciajolo et al. Table 1. Characteristics of the Fuel rapeseed oil (mm2/s)

viscosity at 20 °C viscosity at 150 °C (mm2/s) density at 20 °C (kg/m3) volatilization range (°C)a flash point (°C) elemental analysis, wt % carbon hydrogen oxygen nitrogen aromatics, wt %b

74 5 920 270-480 >290 72.9 11.0 16.0

diesel oil 5 870 90-260 58 85.6 13.8 0.60 30.0

a As measured by thermogravimetric analysis. b As measured by GC-MS analysis.

In spray flames, very few experimental studies on visible-excited lase-induced fluorescence have been carried out and, also in this case, the fluorescence has been attributed to aromatic species on the basis of combined optical and sampling measurements.24-26 In the present study UV-excited fluorescence spectra (λexc ) 266 nm) have been measured in spray flames of a diesel oil and of a vegetable oil (rapeseed oil) produced in a 100 kW three-flux low-NOx burner and compared to fluorescence spectra of high molecular weight species collected in the same optically sampled regions in order to verify the possibility of exploiting the LIF technique to follow fuel aromatic consumption and flame-formed aromatic evolution and with the general purpose of combustion process control. The shape of the LIF spectra and the relative signal intensities have been interpreted on the basis of the chemical and spectroscopic analyses of the condensed species which include unburned fuel and heavy combustion products collected in the regions where LIF spectra have been measured. The LIF and analytical results obtained in a diesel oil spray flame have been compared with those measured in a spray flame produced with an aromatic-free fuel such as a vegetable oil, namely a rapeseed oil, where the formation of aromatic species can be easily followed without interference of aromatic species contained in the original fuel. Experimental Section Burner. Optical and chemical measurements were carried out on spray flames obtained by atomizing the fuels in a 100 kW three-flux low-NOx burner inserted in a cylindrical vertical furnace of ceramic fiber (0.36 m ID and 2.5 m height) equipped with optical accesses. The air excess was kept constant at 20% for each fuel. The air temperature was 20 °C. The differential pressure in combustion chamber was ∆P) +5 mm H2O. The fuels used in this work were rapeseed oil and diesel oil, whose main physical and chemical characteristics are reported in Table 1. The commercial name of the rapeseed oil is “DNS” supplied by NOVANCE (Compiegne; France). The supplier of diesel oil is AGIP. (23) Vander Wal, R. L.; Jensen, K. A.; Choi, M. Y. Combust. Flame 1997, 109, 399-414. (24) Barbella, R.; Beretta, F.; Ciajolo, A.; D’Alessio, A. Polynuclear Aromatic Hydrocarbons; Cooke M., Dennis A. J., Eds.; Battelle Press: Columbus, OH, 1982; pp 83-92. (25) Barbella, R.; Beretta, F.; Ciajolo, A.; D’Alessio, A.; Prati, M. V.; Tamai, R. Proceedings of the Combustion Institute; The Combustion Institute: Pittsburgh, PA, 1988; Vol. 22, pp 1983-1990. (26) Barbella, R.; Beretta, F.; Ciajolo, A.; D’Alessio, A.; Prati, M. V.; Tregrossi, A. Combust. Sci. Technol. 1990, 74, 159-173.

Aromatic Components in a Spray Flame The fuel atomization was obtained by using a commercial mechanical spray nozzle (Danfoss 45°, 1 USgal/h full cone) fed at constant flow rate of 4.5 L/h, measured with a flow meter with 0.1% accuracy. The distribution function of the spray droplet diameter was preliminary measured for each fuel and similar spray and fluid dynamic conditions were obtained by heating the rapeseed oil at 150 °C. At this temperature the viscosity of the rapeseed oil is very similar to that of the diesel oil. The burner could move along the furnace axis so that temperature, sampling, and optical measurements at different flame heights could be performed. More details on the experimental system and flame configurations are reported elsewhere.27-29 Temperature measurements along the flames were carried out by using a fast-response Pt/Pt-13% Rh thermocouple (bead size 150 µm) inserted into the flame for a few hundreds milliseconds in order to avoid soot deposition. The uncertainty of the measured temperature was estimated to be as high as 100 K. Samples. A water-cooled stainless steel probe was used to collect the combustion products along the flame axis. Total hydrocarbons (from C1 to C6) and carbon monoxide were collect in the gaseous phase. The particulate matter, i.e., the solid and liquid material condensed on the probe walls, on a fiber glass filter, and inside cold traps placed along the sampling line, was extracted with dichloromethane to separate the high molecular weight species from the carbonaceous particulate (soot). The soot was washed with dichloromethane until there was no significant fluorescence signal in the washings, to extract the organic species adsorbed on it and then was weighed. The uncertainties of the sampling and the analyses were estimated to be less than (15%. Gas Chromatography (GC). Total hydrocarbons (from C1 to C6) and carbon monoxide in the gaseous phase were analyzed by two gas chromatographs: a HP5890 gas chromatograph, equipped with a 7515 Chrompack Al2O3/KCl 50 m × 0.92 mm (i.d.) capillary column and a flame ionization detector (GC-FID) for the hydrocarbons and a HP5700A gas chromatograph equipped with a 8700 Alltech coaxial column and a thermal conductivity detector (GC-TCD) for carbon monoxide. Gas Chromatography-Mass Spectrometry (GC-MS). The high molecular weight species, dried and weighed, were analyzed by gas chromatography/mass spectrometry (GC/MS) on an HP5890 gas chromatograph, equipped by a HP-5MS cross-linked 5% PhMe siloxane 30 m× 0.25 mm × 0.25 µm film thickness column, coupled with a HP5989A mass spectrometer constituted of an electron impact/chemical ionization (EI/CI) ion source in order to quantify polycyclic aromatic hydrocarbons (PAH) up to 300 u (from naphthalene to coronene). The GC was programmed to operate from an initial temperature of 40 °C (1 min) to 300 °C at 10 K/min. The injector temperature was 280 °C and was operated in splitless mode. UV-Visible Fluorescence Spectroscopy. Emission fluorescence spectra of high molecular weight species dissolved in dichloromethane were measured by a Perkin-Elmer LS-50 B spectrofluorimeter connected to a personal computer (PC). A xenon discharge lamp equivalent to 20 kW for 8 µs duration was used as an excitation light source. The detection device was a gated photomultiplier with modified S5 response for operation to about 650 nm. Monochromators were MonkGillieson type that cover the following ranges: excitation 200(27) Allouis, C.; Romano, M.; Beretta, F. Proceedings of 13th Annual Conference on Liquid Atomisation and Sprays Systems, Florence Italy, 1997; pp 209-215. (28) Allouis, C.; Romano, M.; Beretta, F.; Viegas, L.; D’Alessio, A. Combust. Sci. Tech. 1998, 134, 457-476. (29) Allouis, C.; Apicella B.; Barbella, R.; Beretta, F.; Tregrossi, A.; Ciajolo, A. J. Combust. Technol. Clean Environ. 2000, in press.

Energy & Fuels, Vol. 15, No. 4, 2001 989 800 nm with selectable zero order, emission 200-900 nm with selectable zero order. Excitation spectra are automatically corrected. The wavelength accuracy was (1.0 nm and the wavelength reproducibility was (0.5 nm. Experiments are conducted at ambient temperatures, the usual fluctuations in which are without significant effect on the spectra. Instrumental parameters are controlled by the Fluorescence Data Manager software. The fluorescence measurements of high molecular weight species were performed on very dilute dichloromethane solutions (about 10-20 µg/L) in order to avoid concentration quenching and/or other phenomena that could distort the spectral shape affecting the fluorescence intensity.30 LIF Measurements. LIF measurements have been performed along the flame axis employing the fourth harmonic of a Nd:YAG laser (λexc ) 266 nm) with a pulse duration of 8 ns fwhm. The pulse energy has been chosen in order to avoid photochemical interferences.16 Very low laser energies, 5 MW/ cm2, were used to prevent interferences from-laser induced incandescence (LII), which is negligible below a threshold laser energy.31 Laser-induced fluorescence profiles were obtained by focusing fluorescence signal at 90° from the laser beam onto a 50 µm entrance slit of a Oriel monochromator coupled with an intensified CCD camera. The collection volume was imaged onto the entrance slight of the monochromator with a lens mounted on a translation stage. The position of the lens was adjusted to optimize the collected signal. The intensifier was triggered synchronously with the laser pulse and was opened with a 50-ns gate. A 290 nm high-pass filter was placed before the collecting lens, to avoid the saturation of the CCD camera, due to the laser line elastic scattering. Significant background signals were present, due to the high detector gain. Therefore, each measurement was repeated with the laser blocked and the laser-off data was subtracted from the laser-on data to obtain the final PAH LIF profiles. Then, all the signals have been corrected for the flame background, the filter attenuation, and the response of the detection system.

Results Fuel Characteristics and Composition Profiles. The main physical and chemical characteristics of the diesel oil and of the rapeseed oil used as fuels for this work are reported in Table 1. The rapeseed oil is a vegetable oil typically composed of triglycerides of fatty acids such as oleic, linoleic, and linolenic carboxylic acids. The rapeseed oil and, in general, the vegetable oils are in recent times considered as alternative fuels, pure or in blend with hydrocarbon fuel oils, mainly for diesel engine combustion. However, the high viscosity of vegetable oils, much higher than that of commercial diesel oils (Table 1), strongly limits the atomization quality and spray formation. By consequence, a preliminary treatment of the vegetable oils (transesterification, dilution, etc.) is usually performed in order to lower the viscosity allowing a good atomization of the liquid.32 In this work, the rapeseed oil has been preheated at 150 °C to reach a viscosity similar to that of diesel oil (Table 1) in order to produce similar spray configurations (droplet diameter and distribution).27 It is worth to note that also the volatilities of the two fuels are very different and in particular the rapeseed oil volatilizes in a narrow and much high-temperature (30) Parker, C. A. Photoluminescence of Solutions; Elsevier Publishing Company: Amsterdam, 1968. (31) Ni, T.; Pinson, J. A.; Gupta, S.; Santoro, R. J. Appl. Opt. 1995, 34, 7083-7091. (32) Schwab, A. W.; Bagby, M. O.; Freedman, B. Fuel 1987, 66, 1372-1378.

990

Energy & Fuels, Vol. 15, No. 4, 2001

Ciajolo et al.

Table 2. Concentrations of Soot, High Molecular Weight Species, and PAH in Rapeseed Oil and Diesel Oil Flames rapeseed oil flame height above the burner (mm) soot (mg/Nl) high molecular weight species (mg/Nl) PAHa (mg/Nl) a

100 0.55 15.69 0.032

200 3.16 5.09 1.40

diesel oil flame 300 8.42 4.15 0.99

50 0.55 12.3 3.19

100 1.96 6.81 2.68

150 4.64 8.49 6.21

200 9.3 8.85 4.31

250 9.7 8.06 3.70

As measured by GC-MS analysis of the high molecular weight species.

Figure 1. Axial flame temperature profiles (upper part) and axial carbon monoxide and total hydrocarbons profiles (lower part) of diesel oil and rapeseed oil.

range as also verified by thermogravimetric determination of the fuel volatilisation range. The different fuel volatilities can cause some differences in the evolution of the combustion process and, by consequence, in the flame structure. The differences in the flame structures can be seen in Figure 1 where the flame temperature (upper part) and the composition of CO and total mainly unsaturated light hydrocarbons (HC), which include C1-C6 hydrocarbons, (lower part) are reported for the two flames investigated. It can be noted that the temperature reaches the maximum at the same distance in both flames (about 100 mm height above the burner) that correspond to the end of the spray region where the main fuel oxidation occurs. However, a different level of the flame temperature can be observed possibly due to the above-mentioned differences in the fuel volatilisation characteristics. CO and HC increase after the temperature peak following parallel trends in the same region of the two flames, between 100 and 300 mm height above the burner. The large presence of these species, along with the almost negligible presence of oxygen (about 1 vol %), allow us to label the 100-300 nm region in both flames as a “pyrolysis region” where the maximum formation of PAH and soot occurs. In this region optical and chemical analysis will be preferentially performed. The differences in the temperature and CO and HC concentration levels of the two flames are not important in view of the aim of the work that is to verify the potentialities of the UV-excited laser-induced fluores-

cence techniques for following aromatic fuel components and the flame-formed aromatic species in spray combustion. To this regard the rapeseed oil has been used as a reference free-aromatic fuel in comparison with the diesel fuel that typically contains mono-ring aromatics and polycyclic aromatic compounds in large amounts (Table 1). The PAH contained in the diesel fuel, analyzed by GC/MS, are mainly alkyl derivatives of tworing (naphthalenes) and three-ring (phenanthrenes) aromatics. These species are known to have high absorptivity and fluorescence emission in the UV region as results from room-temperature fluorescence measurements,7-9 consequently UV-excited fluorescence should be very effective for their detection also in flame environment provided that there are no temperature effects on their absorptivity and quantum efficiency. The flame concentration of PAH, has been measured in the rapeseed oil and diesel oil flames by means of GC-MS analysis of the high molecular weight species condensed along the probe and sampling line (trap and filter). The sampling of these species has been performed downstream of the spray in the region between 100 and 300 mm height above the burner where pyrolytic processes preferentially occur (Figure 1). At 50 mm height above the burner the sampling has been also performed and the GC analysis showed the large presence of unburned fuel which constituted the most part of the high molecular weight species condensed along the sampling line. The flame concentration of the most abundant PAH are reported in Figure 2 and Figure 3 for the diesel oil and rapeseed oil flame, respectively. These PAH include unsubstituted two- to seven-ring PAH, but it has to be underlined that a large presence of alkyl-substituted PAH, originating from unburned fuel, has been found at 50 mm of the diesel oil flame, i.e., in the spray/main oxidation region. The sum of the concentration of these alkyl-substituted PAH (mainly methyl, dimethyl, and ethyl PAH derivatives) measured in the diesel oil flame is also reported in Figure 2. Alkyl-substituted PAH were detected also downstream this position and in the rapeseed oil flame, but their contribution to the total PAH was found to be very low. As expected the concentrations of soot, high molecular weight species, and of the total PAH identified in the high molecular weight species, reported in Table 2, are much larger in the diesel oil flame. Nevertheless, the distributions of PAH in the two flames appear very similar irrespective of the fuel used and only an enrichment in the heavier PAH can be noted downstream of the flames. Fluorescence Results. Ultraviolet laser-induced fluorescence spectra (λexc ) 266 nm), measured along the diesel oil and the rapeseed oil flame axis, are reported in the upper and lower part of Figure 4, respectively. The LIF signal, preliminary measured

Aromatic Components in a Spray Flame

Energy & Fuels, Vol. 15, No. 4, 2001 991

Figure 2. Bar diagram of the most abundant PAH concentrations found for diesel oil.

Figure 3. Bar diagram of the most abundant PAH concentrations found for rapeseed oil.

inside the cold spray of the diesel oil and of the rapeseed oil, has been detected only in the diesel oil spray and reported for comparison in the upper part of Figure 4. The cold spray shows a strong fluorescence emission in

the UV range from 280 to 400 nm emission wavelength with a broad peak located between 330 and 340 nm. In the same spectral emission region of the cold spray a lower LIF signal intensity can be observed in the spray

992

Energy & Fuels, Vol. 15, No. 4, 2001

Figure 4. LIF spectra (λexc ) 266 nm) measured in the cold spray of the fuel and in the diesel oil flame (upper part) and LIF spectra measured in the rapeseed oil flame (lower part).

region of the flame, at 50 mm height above the burner, ascribable to residual unburned fuel that can be present in form of droplets and/or vapors. The LIF signal in the UV region completely disappears downstream of the flame and the fluorescence progressively shifts toward the visible with the maximum intensity detected in the 340-500 nm emission region at 150 mm height above the burner. Differently from the diesel oil flame, the UV fluorescence measured in the rapeseed oil flame (Figure 4) is almost negligible whereas visible fluorescence progressively increases and reaches the maximum in the middle of the 100-300 mm flame region. It can be also noted that the visible fluorescence spectrum shows the same shape of the spectrum exhibited downstream of the diesel oil flame (Figure 4). The change in the fluorescence spectral shapes along the diesel oil flame can be better seen in Figure 5 and Figure 6 where the height normalized LIF spectra measured in the spray flame region (Figure 5) and in the PAH formation region (Figure 6) are reported, respectively. The height normalized spectra of the high

Ciajolo et al.

Figure 5. Height normalized LIF spectra (λexc ) 266 nm) measured in the cold spray of the fuel and in the diesel oil flame at 50 mm, 100 mm, 110 mm height above the burner (upper part) and normalized fluorescence spectra (λexc ) 266 nm) of dichloromethane solutions of the raw diesel oil and of the condensed species sampled in the diesel oil flame at 50 mm and 100 mm height above the burner (lower part).

molecular weight species collected in the same optically sampled regions are also reported for comparison in the lower parts of Figure 5 and Figure 6. This representation shows more clearly the gradual shift along the flame from the UV fluorescence, relatively higher in the spray region, toward higher emission wavelengths. However, it can be noted that the LIF fluorescence spectra exhibit, already at 50 mm height above the burner, a significant emission in the visible higher than that measured in the cold spray. The UV fluorescence peak completely disappears after 120 mm height above the burner whereas a broad fluorescence emission in the visible becomes prevalent (Figure 6). The room-temperature fluorescence spectra of the high molecular weight species, dissolved in dichloromethane, have been measured using the same excitation wavelength (λexc ) 266 nm) used for LIF measurements. From 50 mm to 100 mm height above the burner the normalized fluorescence spectra of the high molecular weight species collected in the spray region of the diesel

Aromatic Components in a Spray Flame

Energy & Fuels, Vol. 15, No. 4, 2001 993

Figure 6. Height normalized LIF spectra (λexc ) 266 nm) measured in the diesel oil flame at 120 mm, 150 mm, 200 mm, 300 mm height above the burner (upper part) and normalized fluorescence spectra (λexc ) 266 nm) of dichloromethane solutions of the condensed species sampled in the diesel oil flame at 150 mm, 200 mm, and 250 mm height above the burner (lower part).

Figure 7. Height normalized LIF spectra (λexc ) 266 nm) measured in the rapeseed oil flame at 100 mm, 200 mm, 300 mm height above the burner (upper part) and normalized fluorescence spectra (λexc ) 266 nm) of dichloromethane solutions of the condensed species sampled in the rapeseed oil flame at 100 mm, 200 mm, and 300 mm height above the burner (lower part).

oil flame are very similar to the fuel spectrum. The spectra are peaked in the UV region and show a structured shape with some peaks typical of two-three ring PAH contained in the parent fuel. Consistently with the LIF spectra, also the high molecular weight species exhibit an increase of the visible emission and a decrease of the UV emission starting from the fuel and going from 50 to 100 mm height above the burner. The fluorescence spectra of the high molecular weight species collected in the PAH formation region, from 120 to 300 mm height above the burner, are largely shifted toward the visible and the spectra are much more structured presenting many other peaks in this region. However, the shift toward the visible is not as high as that shown in the LIF spectra, moreover the spectra of high molecular weigh species show a fine structure that is completely absent in the LIF spectra. The height normalized fluorescence spectra measured in the rapeseed oil flame and reported in Figure 7 do not show a relevant fluorescence emission in the UV,

whereas a significant broad emission in the visible region can be observed along the whole flame. LIF measurements of the cold fuel are not reported since, as mentioned before, any significant signal has been detected and this is due to the absence of aromatic fluorescing components in this fuel, as also confirmed by room-temperature fluorescence measurements of the raw rapeseed oil in DCM solution. The fluorescence spectra of high molecular weight species collected in the rapeseed oil flame (Figure 7) exhibit spectral shapes very similar to those relative to the high molecular weight species collected downstream of the diesel oil flame (Figure 6). Discussion In this work UV-LIF techniques, applied to spray flames of liquid fuels, have been coupled and compared with the chemical and fluorescence analysis of the high molecular weight species collected in the same optically

994

Energy & Fuels, Vol. 15, No. 4, 2001

sampled regions of the flames. This has been done in the framework of the aim of this experimental study, that is to explore the potentialities of UV-excited fluorescence techniques for following the fate of aromatic species either originating from unburned fuel or coming from pyrolysis flame reactions. A similar combined approach has been previously used in spray flames 24-26 and in diffusion flames16 by using light sources in the visible where aromatic species are well-known to have a relatively low absorptivity. The use of UV-excitation sources, where aromatics are much more absorbing, should enhance, in line of principle, the sensitivity of the technique extending, at the same time, the range of detection and the possibility of discriminating between the different aromatic ring number systems. Most of the experimental work has been carried out on a diesel oil spray flame where temperature and composition profiles of CO and total light hydrocarbons (from C1 to C6) have been preliminary measured (Figure 1). These profiles are helpful in order (i) to give an overall view of the flame structure, (ii) to individuate the region where optical and chemical sampling should be more favorable for the fluorescence detection of aromatic species. The main spray/oxidation zone ends where the maximum flame temperature occurs and the formation of typical fuel-rich combustion products such as CO and light C1 to C6 hydrocarbons, begins. The region of the maximum formation of CO and total light hydrocarbons is located between 100 and 300 mm height above the burner. In this region the high molecular weight species result to be mainly constituted of flameformed two- to seven-ring PAH as inferred by gas chromatographic/mass spectrometric analysis (Table 2). The distribution of the most abundant PAH produced in the diesel oil flame shows the large presence of alkylsubstituted PAH, originating from unburned fuel in the spray/oxidation region as shown by the chemical analysis of the high molecular weight species collected at 50 mm from the burner (Figure 2). The formation of unsubstituted PAH, already detectable in the spray region at 50 mm, increases along the flame passing through the 100-300 mm pyrolysis zone and reaches the maximum at 150 mm height above the burner in correspondence of the CO and HC concentration peaks (Figure 1). In similar spray and combustion conditions the CO and total HC profiles measured in the rapeseed oil flame exhibit a rise-decay shape similar to that exhibited in the diesel oil spray flame (Figure 1). The rapeseed oil flame has been studied in comparison with the diesel oil flame in order to study the response of the LIF diagnostic to the PAH and aromatic formation in conditions where there is no interference of fuel aromatic components. As previously observed in the diesel flame the formation of PAH occurs in the maximum formation region of CO and total HC (Figure 3). As expected, the formation of soot, high molecular weight species, and of PAH occurs to a lower extent (Table 2) because in the rapeseed oil there are no aromatic components that are known to enhance the formation of soot and PAH in combustion1. The distributions of PAH measured in the diesel oil and in the rapeseed oil flames (Figures 2-3) are very similar and are typically found in fuel-rich combustion

Ciajolo et al.

systems independently on the combustion conditions (premixed and diffusive) and on the fuel characteristics.1 On the basis of the PAH analysis results and of the room temperature fluorescence analysis of the high molecular weight species collected at the same optically sampled flame positions it is possible to attempt the interpretation of the LIF spectral measurements. The shapes of the LIF spectra in the diesel oil flame drastically change along the axis of the flame: the LIF spectra measured in the spray region show a large peak in the UV spectral region (between 320 and 340 nm). This peak occurs in correspondence of the LIF peak measured in the cold fuel spray (upper part of Figure 4). indicating that it is ascribable to unburned fuel aromatic components. Consistently with this attribution, the high molecular weight species collected at 50 mm height above the burner were found to be mainly composed of aliphatic fuel components and alkyl derivatives of small two-, three-ring PAH (Figure 2). This kind of aromatics exhibits the main fluorescence peak just in the spectral region where the LIF peak has been detected.7-9 This is also confirmed by the fluorescence spectra of solutions of the raw fuel that show a prominent fluorescence emission in the UV region (lower part of Figure 5). Moreover, the similarity of raw fuel fluorescence with the fluorescence of the high molecular weight species sampled at 50 mm also reported in the lower part of Figure 5, corroborates the attribution of UV peaked LIF spectra to the unburned fuel aromatic components. The attribution of UV fluorescence to aromatic fuel components is finally confirmed by the complete absence of UV emission in the flame of a free-aromatic fuel like the rapeseed oil (lower part of Figure 4). Surprisingly, the LIF intensity in the UV range strongly decreases along the spray region (Figure 4) and becomes very low just in the maximum PAH formation region of the diesel oil spray flame (from 100 to 300 mm height above the burner) where the maximum UV fluorescence was expected. However, it can be noted that in a height-normalized representation (upper part of Figures 5-6) it is possible to follow the progressive change of the LIF spectra, focused in the UV in the spray region of the flame, and shifting toward the visible in the PAH formation flame region. The disappearance of UV fluorescence as the aromatic fuel depletion occurs and in the PAH formation region indicates that the flame-formed PAH, despite their higher concentration in flame (Table 2 and Figure 2), are much less fluorescent than the fuel PAH. In contrast with this finding the room-temperature fluorescence of high molecular weight species containing either aromatic fuel components or the flame-formed PAH, do not exhibit such drastic changes along the flame. As mentioned before the room temperature fluorescence of high molecular weight species sampled in the spray region is very similar to the fluorescence of the raw diesel oil with the two main peaks located at 330 and 360 nm (lower part of Figure 5). The reduction of the UV emission and a slight shift of the emission toward the visible can be observed in the spray region (Figure 5) due to the depletion of aromatic fuel components and the increasing presence of larger PAH coming

Aromatic Components in a Spray Flame

Energy & Fuels, Vol. 15, No. 4, 2001 995

from pyrolytic reactions. However, in the PAH formation region the main emission peak remain at 360 nm and only a larger emission toward the visible can be observed with the appearance of additional peaks at 400 and 440 nm. Thus, the fluorescence of high molecular weight species in the pyrolytic region of the flame is not so shifted toward the visible as found in the LIF spectra demonstrating that the high molecular weight species, and in particular the flame-formed PAH, do not fluoresce in the same spectral region when their fluorescence is measured in flame conditions. Due to the absence of aromatic fuel components and consistently with the results obtained in the diesel oil flame, it can be noted that the UV component is almost absent in the LIF spectra of the rapeseed oil flame. Moreover, a broad emission feature in the visible region can be observed in the whole PAH formation region (lower part of Figure 4 and Figure 7) very similar to that of the LIF spectra measured in the PAH formation region of the diesel oil flame (Figure 6). The LIF intensity was found lower in the rapeseed oil flame than in the diesel oil flame, consistently with the lower concentration of PAH measured in the rapeseed oil flame (Table 2, Figure 3). The absence of UV emission in the pyrolytic region of both diesel oil and rapeseed oil flames despite the corresponding increase of PAH concentration (Figure 2) along with the corresponding appearance of a large visible emission feature (Figure 4) could be merely due to the fact that the fluorescence due to flame-formed PAH is shifted at higher wavelengths for effect of the high-temperature combustion environment.10,11 In alternative to this hypothesis it can be supposed that the PAH do not fluoresce at all in flame due to the decrease of their quantum efficiency at high temperature, but, unfortunately, experimental data on quantum efficiencies of PAH at high temperatures are not available to support this hypothesis. If this would be the case, responsible for the visible fluorescence could be the heavier unidentified part of the high molecular weight species and indeed it can be observed in Table 2 that total PAH constitute only a fraction of the high molecular weigh species. In the first spray region of the flame the lack of the mass balance is simply due to unburned fuel, but in the pyrolytic region of both the flames the presence of unidentified, possibly large aromatic species not amenable to analyze by GC/MS, has to be hypothesised. In this sense, previous work on the fluorescence characteristics of high molecular weight species sampled in premixed rich flames33,34 has shown that the unidentified fraction is mainly responsible for the visible fluorescence of high molecular weight species. At the same way the high UV emission of aromatic fuel components could be due to the fact that in the spray region the temperature is enough low to favor the

fluorescence emission that is similar in both intensity and spectral shape to that measured in solution. A question arises about the physical state in which the aromatic fuel components are when the strong UV fluorescence is detected in order to establish if the UV fluorescence could be a fingerprint of spray droplets or fuel vapors. The UV fluorescence of aromatic fuel components has been previously measured in the first region of premixed flames of kerosene and gasoline,35 consequently it cannot be at the moment established if the UV fluorescence is due to fuel-rich vapor pockets or to fuel droplets and further work is necessary to this aim. Finally, it has to be underlined that a significant UV component of UV-excited LIF emission spectra has been measured mainly in the relatively low-temperature region of diffusion flames12 whereas a predominant visible feature has been usually found in the pyrolytic regions of premixed flames.34 Consequently, the UVexcited LIF emission spectra measured in the pyrolytic region of the spray flames, studied in this work, suggest that the prevalent regime occurring downstream of the spray region is more typical of a rich premixed regime rather that a diffusive one.

(33) Ciajolo, A.; Barbella, R.; Tregrossi, A.; Bonfanti, L. Proceedings of the Combustion Institute; The Combustion Institute, Pittsburgh, PA, 1998; Vol. 27, pp 1481-1487. (34) Ciajolo, A.; Ragucci, R.; Apicella, B.; Barbella, R.; Tregrossi, A. Chemosphere 2001, 42, 833-839.

EF0100451

Conclusions Laser-induced fluorescence measurements excited in the UV region (266 nm) were performed on a spray combustion system burning a commercial diesel oil and a vegetable aromatic-free fuel like rapeseed oil. The shape of the LIF spectra and their relative intensities have been interpreted on the basis of chemical and fluorescence analyses of the high molecular weight species (unburned fuels and heavy combustion products) collected along the flame axis, in the region where LIF spectra have been measured. The presence of a strong UV fluorescence emission in the spray region of the diesel oil flame, very similar to that measured in the cold fuel spray and in the high molecular weight species there collected, was attributed to unburned aromatic fuel components. The absence of UV fluorescence emission in any region of the flame of a free-aromatic fuel like the rapeseed oil confirmed this hypothesis. A large visible feature of the fluorescence emission was found in the pyrolytic PAH formation region of both the spray flames. The visible emission feature could be assigned to flame-formed PAH species contained in the high molecular weight species hypothesising that their fluorescence spectra are shifted toward the visible for effect of the high temperature flame environment. In alternative to this hypothesis, responsible for visible emission could be the unidentified heavier part of condensed species that are know to fluoresce mainly in the visible range of the emission spectrum.

(35) Fujiwara, K.; Omenetto, N.; Bradshaw, J. B.; Bower, J. N.; Winefordner, J. D. Appl. Spectrosc. 1980, 34, 85-87.