Energy & Fuels 1996, 10, 837-843
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Sources of Naphthalene in Diesel Exhaust Emissions M. M. Rhead* and R. D. Pemberton Department of Environmental Sciences, University of Plymouth, Devon, PL4 8AA, U.K. Received November 10, 1995. Revised Manuscript Received March 13, 1996X
The organic emissions from a diesel engine generally comprise a substantial proportion of hydrocarbon components of fuel-surviving combustion. Accompanying these in the engine exhaust are other components, often identical in structure to the surviving species but which have been produced pyrosynthetically from partially burned fuel. This paper reports on a series of radiochemical spike experiments designed to unequivocally elucidate the degree of survival and the extent of pyrosynthesis of the important fuel component naphthalene under one particular speed and load condition. Emissions were examined after duplicate experiments in which diesel fuel spiked with [14C]naphthalene was combusted in a Perkins Prima DI diesel engine. Diesel exhaust samples were collected using the total exhaust solvent scrubbing apparatus (TESSA) sampling system. Analysis of the exhaust samples was performed by radio-high-performance liquid chromatographic techniques developed for this research. The major radioactive species in the exhaust was identified as [14C]naphthalene, which had survived combustion. The survival represented 0.48% of the original activity. The initial specific activity of the radiochemical in the fuel, for the duplicate experiments, was 1370.60 and 1373.84 mCi/mmol, respectively. Corresponding specific activities of [14C]naphthalene in the exhaust emissions were 327.5 and 317.5 mCi/mmol, respectively. From this, it may be concluded that the contribution to naphthalene recovered in the emissions from naphthalene-surviving combustion was 23.8% while other sources of naphthalene, presumably pyrosynthetic in nature, represented the major proportion (76.2%) of the recovered naphthalene. The sources of the pyrosynthesized naphthalene in the emissions were investigated in a further series of experiments involving both [14C] labeling and a nonlabeled fuel enrichment technique. Fuel spiked with [14C]-2-methylnaphthalene was combusted, and in the exhaust extracts were recovered both radiolabeled naphthalene (0.036% of the original [14C]-2-methylnaphthalene) and radiolabeled 2-methylnaphthalene (0.45% of the original [14C]-2-methylnaphthalene) that had survived combustion. This experiment showed unequivocally that 2-methylnaphthalene was converted to naphthalene in the combustion chamber. Enrichment experiments in which both 1- and 2-methylnaphthalene were added to the fuel just prior to combustion confirmed that demethylation of both species produced naphthalene in small yields (1.79% and 6.80%, respectively, of the naphthalene present in emissions). Reaction mechanisms are suggested for the formation of naphthalene from methylnaphthalene. These include oxidation of the side chain and hydrogenalytic demethylation.
Introduction In recent years in the UK, there has been a substantial increase in the number of new diesel car sales as a proportion of total new car sales. The sale of diesels now account for 20% of new car sales in the UK compared with just 6% in 1990.1 The increased popularity of diesel cars is in part owing to improved engine performance in terms of power output, greater fuel economy, and an improved public image in terms of noise and pollution levels. If the trend in diesel car sales continues, the change in structure of the national car fleet, owing to the increased penetration of diesel, may have important implications for atmospheric pollution, particularly urban air quality2 and human health. Diesel engines have greater emissions of particulate material than corresponding spark ignition (SI) engines. Much of the carcinogenic and mutagenic potential of Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Society of Motor Manufacturers and Traders Ltd, Statistics Department, Motor Industry of Great Britain. World Automotive Statistics. 1993. (2) Diesel Vehicle Emissions and Urban Air Quality. Second Report, ISBN 0 9520771 2 4; Quality of Urban Air Review Group (QUARG), 1993. X
0887-0624/96/2510-0837$12.00/0
diesel exhaust has been attributed3 to the higher levels of particulate emissions. Recent studies2,4 have shown that, in terms of human health, it is the emission of respirable particulate matter less than 10 µm in diameter (PM10) that is especially causing concern. Increased atmospheric burdens of PM10 have been linked with higher mortality rates.4 The health hazards associated with diesel particulates have in part been attributed to a group of compounds known as polycyclic aromatic hydrocarbons (PAH). PAH are present in the solvent organic fraction (SOF) of diesel particulates. Many PAH are proven to be carcinogens.5 Emissions of PAH from diesel engines are significantly greater than from SI engines without catalysts. An increasing proportion of diesel cars on the road may thus affect the atmospheric burden of PAH. While PAH (3) Lewtas, J. Environ. Health Perspect. 1983, 47, 141-152. (4) Pope, C. A.; Schwartz, J.; Ransom, M. R. Arch. Environ. Health 1992, 47, 211-217. (5) Evaluation of Carcinogenic Risks to Humans: Diesel and Gasoline Engine Exhausts and Some Nitroarines; IARC Monographs, World Health Organisation; International Agency for Research on Cancer: Lyon, France, 1989.
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are readily detectable in diesel exhaust,6,7,8 comparatively little research has been undertaken to investigate their origin. An understanding of the origin of PAH in diesel exhaust emissions is fundamental if adequate control of the emission of these compounds is to be achieved. Without this information, control strategies aimed at reducing emissions, in terms of engine design and fuel composition, may be both expensive and inefficient. The majority of PAH in diesel exhaust emissions is thought to result from PAH in the fuel surviving the combustion process.6 A proportion of PAH in the exhaust may be combustion derived. Flame studies have demonstrated the pyrosynthetic formation of PAH from simple aliphatic hydrocarbon species.9 Similarly, it has been demonstrated that PAH may be formed from the pyrolysis of smaller aromatic molecules.10 It has been assumed that the temperature and pressure regime of a diesel combustion chamber would facilitate the formation of PAH from pyrolysis and pyrosynthetic reactions. The most abundant groups of aromatic compounds occurring in diesel fuels are generally naphthalene and alkylnaphthalenes. This research is an endeavor to understand the fate of this group of compounds during combustion in a modern diesel engine. Its fundamental objective is the unequivocal assessment of the sources of naphthalene in the emissions by either survival unburned from fuel or by pyrosynthetic reactions, especially from alkylnaphthalenes. Our approach has been to use [14C] PAH radiotracers added to diesel fuel to investigate the origin of PAH in diesel exhaust emissions. This paper reports on the origin of naphthalene in the exhaust emissions from a Perkins Prima DI diesel engine. In one series of experiments, fuel spiked with radiolabeled [14C]naphthalene was used. In a series of supplementary experiments, fuel spiked with [14C]-2-methylnaphthalene was combusted, [14C]naphthalene recovered in the emissions resulting from demethylation, and [14C]-2-methylnaphthalene recovered in the exhaust were quantitatively assessed. From this information the proportions of naphthalene and 2-methylnaphthalene in the emissions, resulting from both survival and pyrosynthesis, were calculated. Where appropriate radiolabeled precursors could not be commercially obtained, as in the case of 1-methylnaphthalene, the compound was spiked in the fuel with approximately a 4% enrichment. The likely products of its combustion were deduced by a comparison of the emissions from fuel enriched with 1-methylnaphthalene and unspiked fuel combustion.
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The engine used was a direct injection four-cylinder 2L Perkins Prima, mounted on a test bed and controlled by a Borghi & Saveri FA100 eddy current dynamometer, controlled by a test automation series compact controller. The engine has been modified to allow direct injection of the radiotracer
into a single cylinder.11 This has been achieved by modifying the fuel line from the high-pressure pump to the cylinder so that it contains two parallel lines for part of its length. The lines are connected by two three-way valves at both ends with only one line being active at any given time. The inactive line was loaded with the radiotracer by means of a small injection port. The radiochemical was introduced into the cylinder by switching the two three-way valves so that the line containing the radiotracer became the active line. The method for introducing [14C] radiotracers into the Perkins Prima diesel engine has been developed to mimic, as far as is possible, the normal operation of the engine. It was assumed that chemical reactions taking place within the combustion chamber were unaffected by the tiny mass of radiochemical spike added. An A2 diesel fuel was used, with a boiling range of 185-364 °C, specific gravity of 0.863, and an aromatic content of 29%. More detailed fuel specifications are given in Table 1. Fuel consumption during the experiment was determined using a 400 mL graduated measuring buret. Exhaust gas sampling was performed using the total exhaust solvent stripping apparatus (TESSA) sampling system.12 The TESSA system consists of a stainless steel tower connected by a heated transfer line to the single cylinder. The exhaust is sampled using a pressure-controlled countercurrent flow of solvent (dichlorormethane{DCM}/methanol, 1:1), which strips the organic species from the exhaust gases as they pass up the tower. The TESSA system has previously been validated against a standard recommended dilution tunnel system.13 The engine was conditioned at full power for 1 h and then at the test conditions of 2500 rpm and 50 Nm for 15 min prior to sampling the exhaust. A major alteration to the existing sampling procedure has been a lessening in the exhaust sampling time. By reducing the sampling period from 1 min to 25 s, the ratio of radioactive to nonradioactive components in the exhaust is significantly increased, thereby achieving a maximum specific activity of the spike recovered within the exhaust sample. This has reduced the weight of sample, ensuring that it contains sufficient radioactivity to be above the analytical limits of detection and hence has simplified the subsequent analytical procedure. Deuterated compounds (D8-naphthalene, D10-1-methylnaphthalene, D10-phenanthrene), for use as internal standards, were added to the TESSA extract prior to sample workup. The TES was contained in a homogeneous mixture of DCM and methanol. Excess water was added to the mixture to facilitate a separation into an aqueous phase and an organic phase. The mixture was then filtered to remove particulate material and two phases separated by liquid/liquid partition. The aqueous phase was subsequently discarded. The organic phase was dried by adding anhydrous sodium sulfate (5 g) and was filtered and the solvent removed by rotary evaporation (leaving approximately 1 mL). The sample was transferred to a preweighed vial and the DCM removed by gentle nitrogen blow down at ambient temperature. The TES was dissolved in hexane (1 mL) and transferred quantitatively to the top of a slurry-packed silica column. The silica was cleaned by Soxhlet extraction with DCM for 24 h before being fully activated at 185 °C for 12 h. Immediately prior to the separation, the silica was deactivated by injecting a weighed amount of ultrapure water, equivalent to 5% by weight of the adsorbent, and homogenizing for 1 h on a mechanical shaker.14 The ratio of adsorbent to sample weight was 100:1. Separation was achieved by sequential elution with
(6) William’s, P. T.; Abbass, M. K.; Andrews, G. E. Combust.Flame 1989, 75, 1-24. (7) Schuetzle, D. Environ. Health Perspect. 1983, 47, 65-80. (8) Tong, H. Y.; Sweetman, J. A.; Karasek, F. W.; Jellum, E; Thorsrud, A. K. J. Chromatogr. 1984, 312, 183-202. (9) Cole, J. A.; Bittner, J. D.; Longwell, J. P.; Howard, J. B. Combust. Flame 1984, 56, 51-70. (10) Badger, G. M.; Novotny J. Nature 1963, 198, 1086.
(11) Trier, C. J.; Rhead, M. M.; Fussey, D. E.; Ryder, D.; Graham, M. A. Proc.sInst. Mech. Eng. 1991, C433/010. (12) Petch, G. S.; Trier, C. J.; Rhead, M. M.; Fussey, D. E.; Milward, G. E. Proc.sInst. Mech. Eng. 1987, C340/87. (13) Trier, C. J.; Fussey, D. E.; Petch, G. S.; Rhead, M. M. Proc.sInst. Mech. Eng. 1988, C64/88, 135. (14) Later, D. W.; Wilson, B. W.; Lee, M. L. Anal. Chem. 1985, 57, 2979-2984.
Experimental Section
Naphthalene in Exhaust Emissions
Energy & Fuels, Vol. 10, No. 3, 1996 839
Table 1. Fuel Specifications for a Class A2 Diesel Fuel and a Class A2 Diesel Fuel Chemically Modified with 1 and 2-Methylnaphthalene analysis
method
carbon [wt %wt.] hydrogen [wt %] sulfur [wt %] aromatic content mono-ar [wt %] di-ar [%wt.] tri + ar [wt %] total [wt %] cetane no. av cetane no. viscosity at40 °C [cSt] density at15 °C [kg/m3] volatility [°C] IBP [%] 2 5 10 20 30 40 50 60 70 80 90 95 FBP recovery residue loss
flash combustion flash combustion IP373 IP391/mod
ASTM D613 IP71/ASTM D445 IP365 IP123
standard A2 diesel
standard A2 diesel + 4% 1-methylnaphthalene
standard A2 diesel + 4% 2-methylnaphthalene
86.9 ( 0.8 12.2 ( 0.6 0.126
86.9 ( 0.8 12.1,( 0.6 0.158
87.9 ( 0.8 12.3 ( 0.6 0.129
20.7 11.0 2.3 34.0 46.3, 46.5, 47.6 46.8 3.510 862.4
19.9 14.4 2.2 36.5 45.8, 44.8, 45.5 45.4 3.411 867.9
19.8 14.3 2.2 36.3 46.3, 46.3, 46.3 46.3 3.339 868.0
191.0 209.5 222.5 235.5 253.5 267.5 278.0 287.5 298.0 309.5 324.5 345.5 363.5 374.0 98% 1% 1%
185.0 207.5 220.0 233.0 250.5 263.5 274.0 284.5 296.5 309.5 325.0 347.0 367.0 373.5 97.5% 0.5% 2%
188.5 206.5 221.5 233.5 250.5 262.5 273.5 284.5 296.5 309.5 325.0 347.0 365.5 373.5 97.5% 1.0% 1.5%
solvents of increasing polarity. Hexane (15 mL) was used to elute the aliphatic compounds, hexane:ether (15 mL, 10:1 v/v) to elute the nonpolar PAC, and DCM/methanol (15 mL, 1:1 v/v) to remove the more polar PAC.12 The fractions removed were concentrated by rotary evaporation and transferred to clean preweighed vials. The solvent from the individual fractions was removed by nitrogen blow down at ambient temperature. The workup procedures for the analysis of the aromatic compounds contained in the exhaust involve a certain degree of loss. This was evaluated for by quantification of the deuterated internal standards by GC. Linear calibration graphs (R2 not less than 0.995) for the PAH of interest were obtained by triplicate GC analysis of several test mixtures, each containing the relevant pure compounds in concentrations covering the range necessary for the subsequent analysis of the unknown samples. Recoveries of the deuterated internal standards were then used to correct for the experimental losses incurred during the work up procedure.15 A ring size separation of the aromatic fraction by normal phase semipreparative HPLC was facilitated using aminobonded silica as reported previously.15 Normal phase semipreparative HPLC separations were performed using dual pumps (Merck-Hitachi L-6200A intelligent pump and L-6000 LC pump) equipped with a UV-vis spectrophotometer (MerckHitachi L-4200). The nonpolar PAC were separated by ringsize on a Nucleosil-NH2 aminosilane column (10 µm, 10 mm i.d. × 250 mm).16 The solvent gradient consisting of 100% hexane for 20 min changed to 50% DCM over 10 min and held for a further 10 min. Naphthalene was collected in the fraction containing two-ringed PAH eluting between 16 and 20 min. Fractions were collected manually and concentrated by rotary evaporation at reduced pressure. The naphthalene fraction was analyzed by reverse phase analytical HPLC using a binary pump (Merk-Hitachi L-6200A (15) Tancell, P. J.; Rhead, M. M.; Trier, C. J.; Bell, M. A.; Fussey, D. E. Sci. Total Environ. 1995, 162, 179-186. (16) Wise, S. A.; Chester, S. N.; Hertz, H. S.; Hilpert, L. R.; May, W. E. Anal Chem. 1977, 49.
intelligent pump and L-6000 LC pump) equipped with a UVvis spectrophotometer (Merck-Hitachi L-4200) and a fluorescence spectrophotometer (Merck-Hitachi F-1050) in series. Separations were performed on a polymeric C18 SupelcosilPAH column (5 µm, 4.6 mm i.d. × 250 mm, Supelco Inc). A gradient elution program of ACN/water (60/40) to ACN (100%) over 35 min followed by 100% ACN for 10 min was used at a flow rate of 2 mL/min. Retention times for both naphthalene (4.52 min) and 2-methylnaphthalene (7.0 min) were reproducible, allowing identification of both compounds. The method is a variation on standard methods of PAH analysis using reverse phase liquid chromatography17 with the ACN/H2O gradient modified to optimize the separation of the aromatic components in the aromatic subsamples. [14C]naphthalene was received (Amersham International) as a solid (250 µCi) and was dissolved in 1 mL of DCM for determination of specific activity and radiochemical purity. The quoted specific activity of the [14C]naphthalene was 4.5 mCi/ mMol. This compares well with the specific activity of 4.0 mCi/ mMol determined in this laboratory. The radiochemical purity and specific activity were measured by reverse phase high performance liquid chromatography (HPLC) using dual pumps (Merck-Hitachi L-6200A intelligent pump and L-6000 LC pump) using a methanol/water gradient. The radiochemical purity of the [14C]naphthalene was >98%. [14C]-2-Methylnaphthalene was received (Sigma) as a solid (250 µCi). The [14C]2-methylnaphthalene was immediately dissolved in 1 mL of diesel fuel. The radiochemical purity was determined by normal phase semipreparative HPLC using a hexane DCM gradient, followed by reverse phase HPLC using a methanol water gradient, and was found to be >98%. The quoted specific activity of the [14C]-2-methylnaphthalene was 7.1 mCi/ mmol. A specific activity of 5.6 mCi/mmol was determined in this laboratory by liquid scintillation assay. An aliquot of the sample (3 × 10 µL) was made up to 1 mL in toluene, and 10 µL was added to 2 mL of scintillant (butylPBD, 0.5%, in (17) Wise, S. A.; Sander, L. C.; May, W. E. J. Chromatogr. 1993, 329-349.
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toluene). The counting efficiency of the scintillation counter and scintillant matrix (90.5%) was determined by scintillation assay of [14C]hexadecane of known activity. Radioactivity measurement was performed at all HPLC stages using a Berthold LB 506 C-1 detector linked to UV and fluorescence detectors in series. The monitor contained a ytrium glass solid scintillant measuring cell (0.15 mL cell volume). Noise and luminescence events were detected and compensated by using a coincidence circuit with a resolution time of 100 ns. The counting efficiency of the radioactivity monitor was determined by multiple injections of a [14C]naphthalene solution of known activity and was found to be 65%. The detector was remotely controlled from a PC by a Berthold HPLC program. To simulate, as closely as possible, the normal chemical and physical processes taking place in the engine, the [14C]naphthalene was required in a diesel fuel matrix. Accordingly, the DCM solvent was removed by nitrogen blow down at ambient temperature before redissolving the radiochemical in 1 mL of diesel fuel. The specific activity is thereby reduced owing to the dilution of radioactive naphthalene with inactive naphthalene in the unspiked fuel. The specific activity of the naphthalene introduced into the single cylinder was 1371.5 ( 2.12 µCi/mmol for duplicate engine runs. The concentration of naphthalene in diesel fuel was determined by GC-FID. Comparing with a linear calibration graph (R2, 0.998), and accounting for workup losses by reference to the deuterated internal standards, established the concentration of naphthalene in diesel fuel as 1293 mg/L. The mass of naphthalene in the exhaust (26 mg/L) was determined in a similar way. The identity of [14C]naphthalene in the exhaust extracts was confirmed by comparison of the radio HPLC peak, (retention time of 4.57 min) with the retention times for the naphthalene mass peak (4.52 min) and a [14C]naphthalene standard (retention time of 4.57 min). The slight difference in retention times between the mass peak and the radio peak for naphthalene is due to the delay period between the UV detector and radiodetector connected in series. The specific activity of the [14C]-2-methylnaphthalene radiochemical as received (7.5 mCi/mg) was reduced by dilution with 2-methylnaphthalene present in the fuel to 365.2 ( 12.62 µCi/mmol for duplicate engine runs. The mass of 2-methylnaphthalene in both the fuel (4035.83 mg/L) and the emissions (40 mg/L) was determined in a way similar to that for naphthalene in fuel and emissions. The radio HPLC peaks A′ and C′ [Figure 1] were identified as naphthalene and 2-methylnaphthalene, respectively, by the use of appropriate radioactive and nonactive standards. The recovery of [14C]naphthalene as a percentage of the original [14C]-2-methylnaphthalene was 0.036%. A technique has been developed for the combustion of diesel fuel enriched with alkyl aromatic compounds (diesel enriched fuel technique, DEFT). 1-Methylnaphthalene could not be obtained radioactively labeled. The source of 1-methylnaphthalene and its conversion to naphthalene were assessed by this technique in which fuel was enriched with substantial amounts (up to 4%) of 1-methylnaphthalene. The emissions resulting from combustion of enriched fuel were compared with those from the combustion of unenriched fuel. Prior to exhaust sampling, stock solutions (5 L) of 1- and 2-methylnaphthalene (4% w/w) were prepared. The fuel specifications of the modified fuels given in Table 1 show that each enriched diesel fuel had a slightly lower cetane number than the standard fuel. These small differences in cetane numbers would not be expected to alter the combustion characteristics of the fuel significantly. This enriched diesel fuel was added to an auxiliary fuel tank isolated from the main fuel circuit. The fuel supply to the engine was switched to the auxiliary fuel tank. To minimize dilution of the spike in the test fuel with residual fuel in the engine, the fuel return lines were diverted to waste for a period of 3 min. The engine was then allowed to run on the enriched fuel for a further 3 min after which
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Figure 1. (a) Reverse-phase HPLC of the two-ring PAH fraction of diesel exhaust. Chromatographic conditions are the following: Supelcosil-PAH column; mobile phase linear from 60% acetonitrile in water to 100% acetonitrile in 20 min at 1.0 mL/min. UV detection was at 254 nm. Peak A is naphthalene (4.52 min), peak B deuterated naphthalene (4.73 min), and peak C 2-methylnaphthalene (7.00 min). (b) Radiochromatographic separation of the two-ringed PAH fraction of diesel exhaust. Peak A′ is naphthalene (4.57 min) and peak B′ 2-methylnaphthalene (7.13 min). Chromatographic conditions are the same as in (a). duplicate exhaust samples were taken. Prior to the addition of enriched fuel, replicate emissions samples were taken using standard fuel to establish a base line for emissions. An aliquot of fuel was removed immediately after each exhaust sample had been taken to allow the exact concentration of the spike present in the fuel to be determined. The fuel and emissions samples were analyzed by gas chromatography (GC) and gas chromatography-mass spectroscopy (GC-MS).
Results [14C]naphthalene was recovered in diesel exhaust emissions after combustion of fuel spiked with [14C]naphthalene having a specific activity in the fuel burned of 1371.5 ( 0.86 µCi/mmol in duplicate runs. The specific activity of the recovered [14C]naphthalene in the emissions averaged 327.5 ( 14.47 µCi/mmol for the duplicate runs. The reduction in the average specific activity indicated a source of inactive naphthalene, additional to that provided from the fuel, in the exhaust emissions. The [14C]naphthalene surviving combustion was calculated to be 23.8% of the total (surviving and pyrosynthesized) amount of recovered naphthalene. The remaining 76.2% represented naphthalene pyrosynthesized from other components in the fuel. The total recovered naphthalene was 2.03% of the naphthalene in the diesel fuel so that the percentage of naphthalene surviving combustion was 0.484% The diesel combustion of [14C]-2-methylnaphthalene (365 ( 12.62 µCi/mmol) led to the recovery of [14C]naphthalene and [14C]-2-methylnaphthalene (116.87 (
Naphthalene in Exhaust Emissions
67.03 µCi/mmol) in the exhaust emissions. The presence of [14C]naphthalene indicated that demethylation of [14C]-2-methylnaphthalene had occurred during combustion. The radiolabeled naphthalene represented 0.036% of the original activity that went into the combustion chamber. This figure represents the conversion factor of 2-methylnaphthalene to naphthalene in the combustion chamber at the speed and load studied. The observed reduction in the average specific activity for 2-methylnaphthalene indicates an additional source of inactive 2-methylnaphthalene in the exhaust emissions. The total recovered 2-methylnaphthalene (surviving and pyrosynthesized) was 1% of the 2-methylnaphthalene in the diesel fuel. Of the recovered 2-methylnaphthalene, 31.81% resulted from 2-methylnaphthalene surviving combustion and 68.21% was from pyrosynthesized 2-methylnaphthalene. By use of the diesel-enriched fuel technique, 1- and 2-methylnaphthalene were spiked into the fuel at concentrations of 5.35% (46 170 mg/L) and 4.73% (40 810 mg/L), respectively. The total percentage recoveries for 1-methylnaphthalene in exhaust emissions were 1.01% for spiked runs and 0.98% for the standard runs. The total percentage recoveries for 2-methylnaphthalene in exhaust emissions were 1.04% for spiked runs and 1.03% for the standard runs. The similarity of the percentage recoveries for both the spiked runs compared with the standard exhaust runs is a good indication that the addition of a relatively large amount of this particular alkylaromatic compound to the fuel does not perturb the nature of the combustion process. The recoveries of 1- and 2-methylnaphthalene compare well with those of other PAH determined in other laboratories.15,18 The mass of naphthalene collected in the exhaust samples from every liter of standard fuel burned was 26 mg. This increased to 34 mg/L of fuel burnt on combustion of fuel spiked with 1-methylnaphthalene and to 44 mg/L of fuel burnt on combustion of fuel spiked with 2-methylnaphthalene. This represents a conversion factor of 0.017 and 0.044, respectively, for 1- and 2-methylnaphthalene to naphthalene. The conversion factor for 2-methylnaphthalene (0.044) compares well with that calculated for the radiolabeled 2-methylnaphthalene (0.036). This is further evidence that enrichment with substantial addition (4%) of spike produces fates similar to those of components in fuel that are unenriched. The percentage contributions of 1- and 2-methylnaphthalene contained in standard diesel fuel to naphthalene recovered in emissions was 1.79% and 6.80%, respectively. Combining the results from these experiments allows a much clearer picture for the sources of naphthalene in diesel exhaust emissions to be obtained. Of the naphthalene recovered in exhaust emissions under conditions of 2500 rpm and 50 Nm, 24% of this naphthalene came from naphthalene surviving combustion and 76% from pyrosynthesis. Of the 76% pyrosynthesized naphthalene 1.79% came from 1-methylnaphthalene and 6.80% from 2-methylnaphthalene. The remaining 67.41% may be formed from either aliphatic (18) Collier, T.; Rhead, M. M.; Trier, C. J.; Bell, M. A. Fuel 1995, 74 (3), 362-367.
Energy & Fuels, Vol. 10, No. 3, 1996 841
Figure 2. Sources of naphthalene in exhaust emissions of a DI diesel engine under conditions of 2500 rpm and 50 Nm.
species9 and/or smaller aromatic molecules.19 This information is represented in the form of a pie chart in Figure 2. Discussion The purpose of this series of experiments was to determine as accurately as possible the fate of naphthalene and its two monomethyl derivatives during combustion in a diesel engine operated throughout at one speed and load. Experiments carried out by other workers on the relationship between fuel and emissions have failed to make the distinction between the amount of PAH surviving combustion and the amount of PAH being pyrosynthesized during combustion, dealing instead with the total amount of recovered PAH. Williams et al. (1986)20 compared proportions of PAH emissions with those of corresponding fuel PAH. They found that the amounts of certain PAH, for example, phenanthrene and methylphenanthrene, are markedly increased relative to others. These authors20 proposed that these PAH have become more concentrated in the exhaust either by being formed on combustion or owing to their lower combustion efficiencies. They suggested that the two- to four-ring PAH in the exhaust were primarily unburned fuel components. Barbella et al. (1990)21 concede that the pyrolytic formation of PAH will be obscured by the presence of unburned fuel PAH in the emissions from a diesel engine. Both Westerholm and Li (1994)22 and Mitchell et al. (1994)23 have utilized linear regression analysis to distinguish between fuel PAH that have survived combustion and combustionformed PAH. This method accounts only for PAH formed from other non-PAH fuel molecules. The novel radiochemical technique developed at Plymouth University11,15,24 allows the distinction between surviving and pyrosynthesized PAH from both PAH and other organic fuel components to be made when a standard diesel fuel is burned. In the present study [14C]naphthalene in fuel was combusted and the specific activity of the naphthalene in the fuel and corresponding emissions was determined. These results show unequivocally that 24% of the naphthalene recovered (19) Badger, G. M.; Joland, S. D.; Spotswood, T. M. Aust. J. Chem. 1966, 19, 95-105. (20) William’s, P. T.; Bartle, K. D.; Andrews, G. E. Fuel 1986, 65, 1150-1158. (21) Barbella, R.; Bertoli, C.; Ciajolo, A.; D’Anna, A. Combust. Flame 1990, 82, 191-198. (22) Westerholm, R.; Li, H. Environ. Sci. Technol. 1994, 28, 965972. (23) Mitchell, K.; et al. SAE Prepr. 1994 (paper 942053). (24) Rhead, M. M.; Fussey, D. E.; Trier, C. J.; Petch, G. S.; Wood, D. Sci. Total Environ. 1990, 93, 207-214.
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in emissions from the Prima diesel engine has come from fuel-surviving combustion. The remaining 76% of the naphthalene recovered in the exhaust emissions resulted from the pyrosynthesis of naphthalene from other compounds in the fuel during the combustion. Tancell et al. (1995),15 using the same technique, have shown that with the same engine under similar conditions of speed and load, 80% of benzo[R]pyrene recovered in emissions has come from fuel-surviving combustion, while the remaining 20% resulted from pyrosynthesis in the combustion chamber. The sources of the pyrosynthesized naphthalene in the emissions were investigated in a further series of experiments involving both [14C] labeling and a nonlabeled enrichment technique. Fuel spiked with [14C]-2methylnaphthalene was combusted, and both radiolabeled naphthalene (0.036% of the original [14C]-2methylnaphthalene) and 2-methylnaphthalene (0.45% of the original [14C]-2-methylnaphthalene) were found in the exhaust emissions. This demonstrated unequivocally that the formation of naphthalene from 2-methylnaphthalene had occurred during combustion by a dealkylation reaction. Enrichment experiments in which both 1- and 2-methylnaphthalene were added to the fuel just prior to combustion confirmed that demethylation of the species produced naphthalene in small yields (1.79% and 6.80%, respectively) during combustion. Demethylation of methylnaphthalene may have occurred by a number of possible pathways. These routes include oxidation of the side chain and/or hydrogenalytic demethylation. On the basis of work published on the demethylation of toluene, described below, we are suggesting similar mechanisms for the demethylation of naphthalene for which there exsists no comparable literature. The oxidation mechanisms of alkylated PAH and alkylated monocyclics cannot be presumed to be the same, although some similarities would be expected.25 Oxidation of alkylated benzenes has been reviewed by Brezinsky (1986).25 Many of the studies were carried out at 1 atm pressure or lower and thus may not be valid in the context of diesel combustion owing to the much higher pressures involved in the latter. However, Brezinsky (1986)25 postulates that the high pressures involved in combustion in an automobile engine will serve to feed OH radicals into the system, thus promoting an attack on the methyl group. Tully et al. (1981)26 have shown that the most effective attackers of the methyl group on a parent fuel molecule are OH and H. The OH radicals are known to abstract H from the methyl group to produce a benzylic radical. An analogous attack of radicals applied to methylnaphthalene combustion would give reaction 1. Radical-radical
(1)
reactions of benzyl with O atoms and hydroperoxyl radicals, OH2, are responsible for the formation of large amounts of benzaldehyde found during the oxidation of toluene.27 Significantly large amounts of naphthaldehyde and naphthol have been found as products of the (25) Brezinsky, K. Progr. Energy Combust. Sci. 1986, 12, 1-24. (26) Tully, F. P.; Ravishankara, A. R.; Thompson, R. L.; Nicovich, J. M.; Shah, R. C.; Kreutter, N. M.; Wine, P. H. J. Phys. Chem. 1981, 85, 2262.
partial oxidation of 1-methylnaphthalene in the research carried out by Ciajolo et al. (1992)28 in which tetradecane and tetradecane/1-methylnaphthalene underwent combustion in a single cylinder direct injection diesel engine. The benzaldehyde, formed during oxidation of toluene, was attacked by radicals at the site of the easily removable aldehydic H. The benzoyl radical thus formed then easily decomposed to yield CO and the phenyl radical, which could form benzene if a source of H atoms are available.27 The naphthaldehyde formed by partial oxidation of the side of chain of 1-methylnaphthalene may follow an analogous pathway to produce naphthalene. Hydrogenalytic demethylation reactions will involve competition between abstraction of a naphthylbenzylic-H by the H radical and displacement of the methyl group:
(2)
The large concentration of methane produced during the oxidation of toluene supports the presence of the displacement path.25,27 The transition state for the displacement pathway in reaction 2 may be analogous to the ipso-substituted cyclohexandienyl radical found in the demethylation of toluene.29 The intermediate formed from such H-transfers is known to rapidly break down to eliminate the methyl moiety.30 Barbella et al. (1989)31 have indicated that diesel exhaust PAH are slightly enriched in unsubstituted compounds originating from fuel alkylated PAH, which have undergone the oxidation of the alkyl side chain and from pyrolysis reactions. Henderson et al. (1984)32 used selected PAH dissolved in an aliphatic solvent (hexadecane) as fuel in a single cylinder diesel engine to examine relationships among diesel fuel aromaticity, PAH content, and mutagenic activity associated with diesel soot. No naphthalene was detected in the exhaust extracts from the combustion of a single compound fuel containing 1-methylnaphthalene. This may be due to the fact that naphthalene was too volatile for the sampling system (dilution tunnel) to collect. Ciajolo et al. (1992)28 investigated the formation of soot and PAH formation during the combustion of tetradecane and tetradecane/1-methylnaphthalene in a diesel engine. They used fast sampling and chemical analysis of the combustion products collected and have shown naphthalene to be one of the major products. The results reported in this paper are limited to a single speed/load condition of the engine. The contribution of survival and pyrosynthesis to emissions will be affected by driving conditions. Thus, the results ob(27) Brezinsky, K.; Litzinger, T. A.; Glassman I. Int. J. Chem. Kinet. 1984, 16, 1053-1074. (28) Yamada, M.; Amano, A. Pyrolysis Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983; pp 117-132. (29) Mcmillen, D. F.; Malhorta, R.; Chang, S.; Ogier, W. C.; Nigenda, S. E.; Fleming, R. H. Fuel 1987, 66, 1611-1620. (30) Barbella, R.; Ciajolo, A.; D’Anna, A. Fuel 1989, 68, 690-695. (31) Henderson, T. R.; Sun, J. D.; Li, A. P.; Hanson, R. L.; Bechtold, W. E. Environ. Sci. Technol. 1984, 18, 428-434. (32) Ciajolo, A.; D’Anna, A.; Barbella, R. Combust. Sci. Technol. 1992, 87, 127-137.
Naphthalene in Exhaust Emissions
tained for the contribution of pyrosynthesis and survival to total recovered naphthalene would be expected to change with differing driving conditions. For example, at low load the level of recovered naphthalene would be high owing to the lower overall combustion temperatures. The extent of pyrosynthesis may be influenced by the load, with low load (characterized by a wide range of temperatures for a longer time) favoring pyrosynthetic production of naphthalene (Collier et al., 1995). At high load the opposite trend would be expected, i.e., reduced levels of recovered naphthalene, with pyrosynthesis making a smaller contribution to the total amount of recovered naphthalene. The use of radiochemical and fuel-enriched experiments has produced a valuable insight into the fate of naphthalene during combustion and its sources in emissions. The fate of naphthalene during combustion can be divided into three routes: conversion to CO2, conversion to other hydrocarbon species (pyrosynthesis), and survival. By use of a radiolabeled technique, the proportion of naphthalene contained in fuel that survives combustion in a diesel engine at midspeed and midload has been unequivocally determined (0.484%). The conversion to CO2 and pyrosynthesis may be
Energy & Fuels, Vol. 10, No. 3, 1996 843
grouped together to produce the remainder (99.516%), with CO2 presumably representing the larger portion of this amount. The naphthalene present in emissions was from two sources: naphthalene surviving combustion (24%) and naphthalene pyrosynthesized from other hydrocarbons (76%). Of the naphthalene pyrosynthesized during combustion, 8.59% came from the 1- and 2-methylnaphthalene isomers. The large number of diand trimethyl isomers in the fuel may represent a significant source for the remaining 67.41% of the pyrosynthesized naphthalene in the emissions. Alternatively, pyrosynthesis of PAH from small unsaturated hydrocarbons, as suggested by the early work of Badger and Novotny,10 may be significant. Acknowledgment. The authors gratefully acknowledge financial support from the UK Engineering and Physical Sciences Research Council (EPSRC), the Ford Motor Corporation (U.S.), and Perkins Technology Limited, Peterborough (UK). The authors also thank Dr. Paul Tancell for his help and advice. EF9502261
