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New Approach for Methane Conversion Using an rf Discharge Reactor. 2. Characteristic of Polycyclic Aromatic Hydrocarbon Emissions Cheng-Hsien Tsai,*,† Kuo-Lin Huang,‡ Lien-Te Hsieh,‡ How-Ran Chao,‡ and Kuan-Chuan Fang§ Department of Chemical and Material Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 807, Taiwan, Department of Environmental Engineering and Science, National Pingtung University of Science and Technology, Pingtung, Taiwan, and Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan
The previous study demonstrated that the radio frequency plasma-oxidized approach could successfully convert methane/oxygen mainly into syngas. However, the trace toxic polycyclic aromatic hydrocarbons (PAHs) were obtained and examined in this study. To convert pure CH4 produced the maximum amounts of PAHs, reaching 15.0 µg/g of CH4 converted, at 90 W and 1.33 kPa. However, elevating the inlet O2/CH4 molar ratio to 1.5, the minimum PAH emission rate was achieved, to be 2.46 µg/g of CH4 converted at 90 W and 4 kPa. In addition, the emission characteristics of individual PAH mass, the PAH homologues, and the toxicity were also discussed. Moreover, the association of acetylene selectivity with PAH mass was fairly consistent, to reveal that the acetylene addition mechanisms might play an important role in the plasma conversion system. Introduction In a high-temperature environment, the low methane conversion or fast formation of CO2 and H2O often limits the routes for directly converting methane into the liquid products, such as methanol and formaldehyde.1-3 Hence, methane is usually converted into syngas (H2 + CO mixture) first, and then syngas is used for the synthesis of methanol, ammonia, alcohols, aldehydes, or gasoline.4 The syngas production from methane is usually through steam reforming, partial oxidation, and carbon dioxide reforming approaches. These traditional catalytic methods usually operate at relative high pressure, high temperature, or both, and yield unwanted byproducts, such as CO2 and H2O.5-7 Except for the traditional methods, the plasma process has also been used to convert methane into C2 hydrocarbons (ethane, ethylene, acetylene), such as the 27-MHz radio frequency (rf) plasma or the pulsed highfrequency plasma.8-11 However, some trace solid, toxic byproducts or polymerized film has been found: first, carbon depositions, CO and H2 formed for producing C2 hydrocarbons in the CH4 microwave plasma;12,13 second, to form polycyclic aromatic hydrocarbons (PAHs), CO2, and C2H2 though the nucleation and growth of benzene and solid carbon via the thermal decomposition of CH4 into H2 and carbon black using a dc plasma;14 third, the formation of alkanes, alkenes, oxygenates, solid carbonaceous species, and syngas when used a CH4/CO2 dielectric-barrier discharges (DBDs) approach to produce C2-C4 hydrocarbons;15-17 fourth, the formation of H2O, C2H4, C2H6, C3H8, trace amounts of C3-C6, * To whom correspondence should be addressed. Tel.: +8867-381-4526 ext. 5110. Fax: +886-7-383-0674. E-mail: chtsai@ cc.kuas.edu.tw. † National Kaohsiung University of Applied Sciences. ‡ National Pingtung University of Science and Technology. § National Cheng Kung University.
plasma-polymerized films, and oxygenates though the DBDs in order to convert CH4/CO2 into syngas.16,18 However, the byproducts are not yet identified very clearly in the plasma system, especially the toxic PAHs. PAHs are one of the first identified airborne carcinogenic pollutants and, hence, are important in health effects. Traditionally, PAHs and their derivatives are associated with the incomplete combustion of simple fuels such as methane and benzene or pyrolysis of simple hydrocarbons arising mostly from anthropogenic activities such as the burning of gasoline in motor vehicles and industrial production activities.19,20 The formation of the first aromatic ring is usually described by the reactions of small hydrocarbon species emerging from the pyrolysis of fuel.21,22 The further growth to PAHs is commonly by the successive addition of not only species containing an even number of carbon atoms, such as C2Hx and C4Hy, but also the species containing an odd number of carbon atoms, such as CHi and C3Hi molecules, following the dehydrogenation reactions.23,24 In addition, the acetylene addition mechanism is significantly associated with benzene, naphthalene, and further PAHs and soot formation.25 Hence, the emission profiles of acetylene are important. Our preliminary study has successfully demonstrated the application of syngas production from CH4/O2 mixtures using the 13.56-MHz rf plasma approach.26 Homogeneous discharges make rf plasma potential with a single-stage, noncatalytic syngas production process. Moreover, the use of external electrodes avoids the erosion and contamination of the electrode and the catalyst poisons. However, CH3 radicals naturally are abundant in the rf plasma due to the discharge and can be applied to generate energetic electrons to impact with CH4 and O2 to form a large amount of free radicals, including CH3, CH2, CH, H, and O.8,27-30 CH3 radicals are important building blocks for the subsequent production of C2H5, C3H4, and C5H5 via reaction with CH3,
10.1021/ie050124q CCC: $30.25 © 2005 American Chemical Society Published on Web 07/22/2005
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C2H2, and C4H2, and they lead to the formation of benzene and subsequent PAHs.24 Therefore, in the CH4/ O2 rf plasma reactor, the conditions for reaching the maximum methane conversion and syngas selectivity have been suggested; however, the characteristics of PAH emissions and the conditions to yield the minimum toxic PAHs have not yet been discussed and will be carried out in this study. Experimental Section The same experimental setup that was used to convert methane/oxygen into syngas was described in detail in our previous study.26 Briefly, the reactants, included CH4, O2, and balance N2, which were supplied from compressed gas cylinders; their flow rates were adjusted using a calibrated mass flow controller and they were introduced into a gas mixer. The mixtures entered a vertical cylindrical glass reactor (4.14-cm i.d., 15-cm length). The reactor wrapped two outer, symmetrical, 5.4-cm-height copper electrodes that coupled to a 13.56-MHz rf generator (PFG 600, Fritz Huttinger Elektronik Gmbh) with a matching network (Matchbox PFM) to generate discharges. All experiments were carried at the following conditions: inlet O2/CH4 molar ratio (R) ) 0-1.5, applied rf power (W) ) 30 or 90 W, system pressure (P) ) 1.33 or 4 kPa, at a total flow rate of 200 standard cm3/min, temperature of feeds 303 K, and feeding concentration of methane 5%. To avoid introducing contaminants into the vacuum system and to test for leaks, before any experiment was conducted, the pressure was pumped until lower than 1.3 Pa by attaching a mechanical vacuum pump (Pfeiffer, DUO 065 dc). When the plasma conversion process was performed, N2 was passed through the overall system for at least 15 min to clean up the reactor. PAHs and Byproduct Analysis. A modified experimental setup was designed to collect the PAH samples. The rear of the reactor was connected to the sampling pipe filled with a tube-type glass fiber filter (25 × 90 mm, Whatman glass fiber thimble), which was used to collect particulate PAHs. The samples were stored in a prebaked glass bottle that was wrapped with aluminum foil for shipment before the chemical analysis. The sampler then linked to the glass cartridge, packed with a 5-cm polyurethane foam (PUF) plug, followed by a 2.5cm XAD-16 resin supported by a 2.5-cm PUF plug, to collect the gaseous PAHs and stored in a clean screwcapped jar with a Teflon cap liner for transportation. PAHs analysis followed the previous study.31 Each collected sample (including particulate and gaseous PAH samples) was extracted in a Soxhlet extractor with a mixed solvent (n-hexane/dichloromethane; v/v, 1:1; 500 mL each) for 24 h. The extract was then concentrated, cleaned up, and reconcentrated to exactly 1.0 mL. PAH contents were determined by a gas chromatograph (GC) (Hewlett-Packard 5890A) equipped with a capillary column (HP Ultra 2, 50 m × 0.32 mm × 0.17 µm), a mass-selective detector (MSD) (Hewlett-Packard 5972), and an automatic sampler (HP-7673A). This GC/MSD operated under the following conditions: injection volume 1 µL; splitless injection 310 °C; ion source temperature at 310 °C; oven temperature from 50 to 100 °C at 20 °C/min, 100 to 290 °C at 3 °C/min, and then hold at 290 °C for 40 min. The masses of primary and secondary ions of PAHs were determined using the scan mode for pure PAH standards. PAHs were qualified using the selected ion monitoring mode. The concentrations of 21
PAH species were determined: including naphthalene (Nap), acenaphthylene (AcPy), acenaphthene (Acp), fluorene (Flu), phenanthrene (PA), anthracene (Ant), fluoranthene (FL), pyrene (Pyr), cyclopenta[c,d]pyrene (CYC), benzo[a]anthracene (BaA), chrysene (CHR), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[e]pyrene (BeP), benzo[a]pyrene (BaP), perylene (PER), indeno[l,2,3,-cd]pyrene (IND), dibenz[a,h]anthracene (DBA), benzo[b]chrycene (BbC), benzo[ghi]perylene (BghiP), and coronene (COR). The GC/MSD was calibrated with a diluted standard solution of 16 PAH compounds (PAHs mixture-610M; Supelco) plus 5 additional individual PAHs (Merck, Darmstadt, Germany). In addition, CH4, CO, CO2, H2, C2H2, C2H4, and C2H6 were also quantified by a gas chromatograph (Varian, GC3800; column is Supelco 13821, 10 ft × 1/8 in., packing 100/120 carbonsive S-II) equipped with a thermal conductivity detector. Results and Discussion In this study, the total PAH emission rate (mass) was regarded as the sum of the quantities of 21 PAH compounds in both gaseous phase and particulate phase for each collected sample (with a unit of µg of PAHs/(g of CH4 converted). Comparison of Total PAH Mass Emitted between Different Operating Factors. Table 1 shows the mean of individual PAHs and total PAH emission rates of that contained in the effluents at six different conditions. The results revealed that inlet O2/CH4 molar ratio (R) was the most important with the operating pressure (P) the next factor that affected the magnitude of total PAH emission rates. The applied power (W) had no effect on PAH emission rates. First, comparasion of the PAH emission rates at R ) 0.5 (S5), 1 (S7), and 1.5 (S8) at 4 kPa and 90 W showed that the PAH masses reduced from 7.51 to 2.46 µg/g of CH4 converted. Less PAHs were emitted at a higher R due to the large amount of oxygen in the plasma, which oxidized the methane more completely and led to reduction of the concentrations of hydrocarbons and free radicals, such as C2H2, CH3, CH2, and CH, as well as yielding a large amount of CO and CO2. Hence, the formation of PAHs and soot is inhibited strongly, syngas production via plasma process should be operated at an R as high as possible. Besides, at R ) 0 (S2), that is, no oxygen added in the methane plasma, the PAH emission rate reached the maximum value of 15.03 µg/g of CH4 converted, to be significantly higher than at R ) 0.5 (S3, 5.20 µg/g of CH4 converted) at 1.33 kPa and 90 W. Hence, to reform pure methane mainly into C2-hydrocarbons regardless of the applied power and pressure, by using the rf plasma approach, will yield high amounts of PAH mass rather than convert methane/ oxygen into syngas. Moreover, a lesser methane conversion was found at a lower R.26 The results indicate that methane is not suitable for conversion into syngas at a lower R, especially at R ) 0. That is to say, to convert methane/oxygen into syngas is a feasible conversion process rather than converting methane into hydrocarbons if utilizing the rf plasma technique. Second, the comparisons of PAH emission rate, between 30 and 90 W under 1.33 kPa with R ) 0 (S1 vs S2) or 4 kPa with R ) 1 (S6 vs S7) conditions, revealed that the effect of the applied power on the PAH yields was not evident, though the reactor operating at a lower power seemed to produce less PAHs. The PAH mass
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Table 1. Mean PAHs Emission Rates for Both Gaseous Phase and Particulate Phase (µg/g of CH4 Converted) Emitted from the CH4/O2/N2 rf Plasma Conversion Process (Fixed Conditions: [CH4]in ) 5%) conditions
S1
S2
S3
S4
operating pressure (kPa) applied rf power (W) inlet O2/CH4 molar ratio)
1.33 30 0
1.33 90 0
1.33 90 0.5
2.66 30 0.5
4 90 0.5
4 30 1
4 90 1
4 90 1.5
Nap AcPy AcP Flu PA Ant FL Pyr CYC BaA CHR BbF BkF BeP BaP PER IND DBA BbC BghiP COR
9.74 0.184 0.567 0.608 1.142 0.096 0.287 0.255 0.005 0.009 0.024 0.444 0.400 0.139 0.015 0.014 0.015 0.003 0.060 0.012 0.158
11.09 0.155 0.576 0.634 0.845 0.083 0.213 0.144 0.061 0.006 0.020 0.395 0.140 0.113 0.056 0.020 0.032 0.000 0.297 0.000 0.149
3.52 0.031 0.105 0.268 0.483 0.025 0.148 0.085 0.014 0.019 0.000 0.179 0.067 0.069 0.017 0.052 0.008 0.010 0.060 0.005 0.043
4.78 0.104 0.389 0.381 0.410 0.029 0.169 0.075 0.034 0.016 0.017 0.138 0.092 0.054 0.021 0.072 0.003 0.006 0.053 0.011 0.069
4.79 0.103 0.328 0.164 0.332 0.123 0.356 0.107 0.064 0.013 0.058 0.263 0.263 0.112 0.051 0.083 0.013 0.020 0.232 0.020 0.014
2.59 0.117 0.157 0.131 0.251 0.077 0.051 0.048 0.054 0.055 0.065 0.174 0.196 0.079 0.040 0.095 0.040 0.011 0.131 0.031 0.041
2.53 0.103 0.213 0.105 0.305 0.096 0.108 0.090 0.061 0.067 0.078 0.213 0.234 0.109 0.040 0.071 0.063 0.014 0.199 0.045 0.013
1.44 0.060 0.080 0.063 0.064 0.057 0.038 0.037 0.031 0.026 0.031 0.106 0.120 0.090 0.025 0.035 0.020 0.007 0.104 0.015 0.006
5.06 0.11 0.30 0.29 0.48 0.07 0.17 0.11 0.04 0.03 0.04 0.24 0.19 0.10 0.03 0.06 0.02 0.01 0.14 0.02 0.06
total
14.18
15.00
5.20
6.92
7.51
4.43
4.75
2.46
7.56
decreased only from 15.0 to 14.18 µg/g of CH4 (S2 f S1) converted and from 4.75 to 4.43 µg/g of CH4 converted (S7 f S6) when the W was reduced from 90 to 30 W. Because at a higher power, a higher electron density decomposes more methane, and then yields more CHx fragments, to form more C2H2 molecules to lead to the elevation of the acetylene addition reactions. Usually, a high-temperature combustion process contributed less PAH mass in the flue gas. For example, the fueled-powered engine operated at a higher working temperature, that is, at a higher engine load or speed, emitted lower PAH concentrations in the engine exhaust.32,33 However, though a higher plasma density accompanies a higher effluent temperature, the measured effluent temperature in the CH4/O2 plasma conversion process was lower than 337 K and did not cause noticeable effects on PAH formation. Third, comparison of PAH mass between various P, such as S3 (1.33 kPa), S4 (2.66 kPa), and S5 (4 kPa), at R ) 0.5, showed that the PAH emission rate was affected slightly by the operating pressure and decreased from 7.51 to 5.20 µg/g of CH4 converted if P was reduced from 4 to 1.33 kPa. The plasma operated at a lower pressure in conjunction with a higher mean electron energy conducted to the methane/oxygen mixtures and could be dissociated into a large amount of active species to not only reform mainly into H2, CO, CO2, and H2O but also inhibit the C2H2 production and keep from subsequent formation of PAHs. Moreover, a lower pressure resulted in a lower gas density to reduce the probability of complex molecule combination. Relationship of Acetylene with PAH Mass Emitted. Because C2H2 is a species that is generally believed to be the key precursor to the formation of aromatics, PAHs, and soot; the C2H2 selectivity (%), defined as C2H2 produced/(CH4 consumed × 2) × 100%, is examined and compared. Similar with the trends of PAH emission rate, the C2H2 selectivity decreased with elevated R, and decreased W or P. Figure 1 showed that as R increased from 0.5 f 1.0 f 1.5, the selectivities of C2H2 reduced from 2.16 (S5) f 1.21 (S6) f 0.87% (S8)
S5
S6
S7
S8
av
at 90 W and 4 kPa, from 1.62 f 0.97 (S6) f 0.66% at 30 W and 4 kPa, and from 0.72 (S3) f 0.55 f 0.27% at 90 W and 1.33 kPa. The profiles of C2H2 selectivity are consistent with the PAH emission rates; however, the relationship between C2H2 selectivity and PAH mass is not very clear, warranting the need for further confirmation. However, the results still support the notion that the methane/oxygen plasma conversion process can produce less of both C2H2 and PAHs at the conditions of lower P or W and higher R. In rf plasma, the abundant CH3 and CH radicals play important roles in the formation of C2H2. The acetylene can be formed via CH3 precursors described by a freeradical mechanism and preceded by stepwise dehydrogenation reactions CH4 f CH3 f C2H6 f C2H4 f C2H2.29,34-36 In addition, acetylene formation via the direct coupling of the CH radicals yielded from CH4 dissociation or atomic carbon combined with H has also been proposed in pulsed dc, rf, microwave, or corona plasmas.37-39 Whatever the C2H2 formation mechanism, when elevated R and decreased P and W result in the elevation of oxidants and electron mean energy, as well as reduction of electron density, then all favor the oxidation reactions and inhibit the formation of C2H2. Characteristics of Individual PAH Mass with Toxicity. Several PAH compounds are known human carcinogens, and their carcinogenic potencies are as-
Figure 1. Selectivities of acetylene (%) at various conditions.
Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 6569 Table 2. PAH Compounds and Their TEFs40,41
a
PAHs
TEF
PAHs
TEF
Nap AcPy Acp Flu PA Ant FL Pyr CYC BaA CHR
0.001 0.001 0.001 0.001 0.001 0.01 0.001 0.001 a 0.1 0.01
BbF BkF BeP BaP PER IND DBA BbC BghiP COR
0.1 0.1 a 1 a 0.1 1 a 0.01 a
No TEF has been suggested.
sessed on the basis of BaP as a reference compound by assigning the toxic equivalent factor (TEF) of BaP as 1.0, to represent the relative carcinogenic potency of the given PAH compound. Table 2 lists of TEF of individual PAHs.40,41 To understand the emission characteristics of carcinogenic PAHs, the species emitted from the plasma process were examined. Table 1 shows that the most abundant compounds are lower toxic 2- or 3-ring compounds, including NaP, PA, AcP, and Flu with the mean emission rates of 5.06, 0.48, 0.30, and 0.29 µg/g of CH4 converted, respectively. These PAH compounds have lower carcinogenic risks due to their TEF of 0.001. The emission rates of the highest carcinogenic PAHs, including BaP and DBA, averaged 0.03 and 0.01 µg/g of CH4 converted, respectively, were much lower than lower carcinogenic PAHs. The results are slightly different with the traditional industrial processes that produce a large amount of 2-, 4-, and 5-ring PAHs. The fuel combustion process revealed that NaP, AcP, CYC (5-ring), and BbF (5-ring) were the major species in the exhaust of gasoline with additives-powered engines,42 and abundant NaP, PA, FL (4-ring), Pyr (4-ring), BaA (4-ring), and CYC (5-ring) were found in the particle-bound PAHs from a heavyduty diesel-powered engine.32 The rf plasma process yields mainly 2- or 3-ring PAHs because the main components in the discharge zone are dissociated fragments except for reactants, as well as in the afterglow
zone, the active species with very short quenching time and lifetime at a low-pressure environment limit the synthesis probabilities of the complex structure of PAHs. Comparison of Mass Fraction of PAH Homologue. PAH homologues grouped by the number of rings are 2-ring (Nap), 3-ring (AcPy, Acp, Flu, Ant, PA), 4-ring (FL, Pyr, BaA, CHR), 5-ring (CYC, BbF, BkF, BeP, BaP, PER, DBA, BbC), 6-ring (IND, BghiP), and 7-ring (COR). Because the larger ring PAHs are often more carcinogenic, the PAH emission characteristics are examined according to ring number. Table 3 lists the calculated results of the total PAH emission and shows that the averages of PAH homologue emission rates are of the following order: 2-ring (64.2%) > 3-ring (16.4%) > 5 ring (12.9%) > 4-ring (5.0%) > 6 + 7-ring (1.5%). Interestingly, except for the more 5-ring PAH homologues found, the orders and fractions of other PAH homologues from this rf plasma process are similar to those from the fuel-powered engine. PAH emission from the gasoline additives-powered engines showed that the mean fraction of total PAH emissions contributed by the PAH homologues were 2-ring (63.3%) > 3-ring (17.6%) > 4-ring (8.31%) > 5-ring (0.45%) > 6 + 7-ring (0.23%),42 showing the same order of a heavy-duty diesel-powered engine.32 More fractions of 5-ring PAH homologues were produced because of the higher emission rates of BbF, BkF, and BdC, averaging in the range of 0.14-0.24 µg/g of CH4 converted. The 5-ring PAHs may be formed via the direct formation pathways in the discharge environment, such as the reactive coagulation pathway: 2C10H8 (2-ring) f C20H12 (5-ring) + 4H.43 However, it should be noted that to infer the formation mechanisms of 5-ring PAHs is very difficult in the complicated plasma system at this stage and it needs to be studied further. To assess PAHs homologue distribution, we further classified total PAHs into three categories: low molecular weight (LM-PAHs, containing 2-3-ring PAHs), middle molecular weight (MM-PAHs, containing 4-ring PAHs), and high molecular weight (HM-PAHs, containing 5-7-ring PAHs). Table 3 shows that the magnitudes of PAH homologues and shared the same trends: LM-PAHs (70.4% ∼ 89.1%) > HM-PAHs (8.4% ∼ 22.7%)
Table 3. Fraction of PAH homologue Mass (%) Counted for the Total PAH Mass conditions
S1
S2
S3
S4
S5
operating pressure (kPa) rf power (W) inlet O2/CH4 molar ratio (R)
1.33 30 0
1.33 90 0
1.33 90 0.5
2.66 30 0.5
4 90 0.5
4 30 1
S6
4 90 1
S7
4 90 1.5
S8
2-ring (%) 3-ring (%) 4-ring (%) 5-ring (%) 6-ring (%) 7-ring (%) LM-PAHs (2+3-ring) (%) MM-PAHs (4-ring) (%) HM-PAHs (5+6+7-ring) (%)
68.7 18.3 4.06 7.61 0.19 1.11 87.0 4.06 8.91
73.8 15.3 2.55 7.19 0.22 0.99 89.1 2.55 8.40
67.6 17.5 4.83 8.96 0.25 0.83 85.1 4.83 10.1
69.0 19.0 4.00 6.79 0.21 0.99 88.0 4.00 7.99
63.8 14.0 7.12 14.5 0.43 0.19 77.8 7.12 15.1
58.4 16.5 4.95 17.6 1.61 0.93 74.9 4.95 20.1
53.1 17.3 7.22 19.8 2.28 0.28 70.4 7.22 22.4
58.7 13.2 5.37 21.1 1.41 0.26 71.9 5.37 22.7
av
64.2 16.4 5.0 12.9 0.8 0.7 80.5 5.0 14.5
Table 4. Distributions of PAH Mass in Gaseous Phase and Particulate Phase (%) Contained In LM-PAHs, MM-PAHs, HM-PAHs, and Total PAHs at Different Conditions conditions
S1
S2
S3
S4
S5
S6
S7
S8
operating pressure (kPa) rf power (W) inlet O2/CH4 molar ratio (R) distribution (%) LM-PAHs MM-PAHs HM-PAHs total PAHs
1.33 30 0 gas par 92.3 7.7 69.6 30.4 59.2 40.8 88.4 11.6
1.33 90 0 gas par 92.1 7.9 91.7 8.3 50.8 49.2 88.6 11.4
1.33 90 0.5 gas par 90.7 9.3 85.0 15.0 66.2 33.8 88.0 12.0
2.66 30 0.5 gas par 93.0 7.0 91.3 8.7 70.8 29.2 91.2 8.8
4 90 0.5 gas par 92.0 8.0 85.9 14.1 88.7 11.3 91.0 9.0
4 30 1
4 90 1
4 90 1.5 gas par 90.3 9.7 85.3 14.7 90.6 9.4 90.1 9.9
gas 93.1 89.1 88.6 92.0
par 6.9 10.9 11.4 8.0
gas 90.0 83.8 90.4 89.6
par 10.0 16.2 9.6 10.4
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> MM-PAHs (2.55% ∼ 7.22%). The trends are also similar to the distribution of PAH homologues, LM-PAHs (70.0-95.6%) > HM-PAHs (2.89-19.9%) > MM-PAHs (1.54-10.0%), in the stack flue gases of various industrial processes, including blast furnace, basic oxygen furnace, coke oven, electric arc furnace, heavy oil plant, power plant, and cement plant.44 The results suggested that the formation pathways of PAHs in rf plasma might be similar to the above industrial processes, while still different from the combustion of heavy oil, which produced considerably high amounts of 4-7-ring PAHs.44 However, it should be noted that to identify the formation mechanisms of PAHs in a plasma reactor is still difficult at this stage. Higher fractions of LM-PAHs produced at lower pressure were found. Table 3 showed that, at 1.33 kPa, the mass fractions of LM-PAHs equaled 85.1% (S3), apparently greater than 77.8% (S5) at 4 kPa. Due to a lower pressure, the lower gas density reduced the combination probabilities of complex structure of HM-PAHs. In addition, the mass fractions of LM-PAHs at R ) 0.5 (77.8%, S5) were higher than that at R ) 1 (70.4%, S7) or 1.5 (71.9%, S8). Results showed that the higher fractions of LM-PAH mass could be obtained at a lower P and a lower R to reduce the emission toxicity. Table 4 shows the individual distribution of gaseous PAHs and particulate PAHs (%) contained in LM-PAHs, MM-PAHs, HM-PAHs, and total PAHs. For total PAHs, the fractions of gaseous PAHs (range, 88.0-92.0%) were consistently higher than the fractions of particulate PAHs (range, 8.0-12.0%); to reveal the distribution of PAHs between gas phase and particulate phase, total PAHs apparently were not affected by the different operating factors. Similar results also were found for the LM-PAHs and HM-PAHs. In addition, the results also suggested PAHs may be synthesized primarily in gas phase; the control of gaseous PAH emissions would be more important than that of particulate PAHs. Interestingly, to examine the HM-PAH homologues, especially at a low pressure of 1.33 Pa, the fractions of particulate PAHs (range 33.8-49.2%, for S1-S3) were significantly higher than that at 4 kPa (range 9.4-11.4%, for S5-S8); therefore PAHs on the particulate phase should be removed further because the HM-PAHs associated with the higher TEFs are more carcinogenic, though to operate at a lower P could yield less total PAH mass at the same conditions. Conclusion Our previous study suggested that the better operating conditions were at an inlet O2/CH4 molar ratio (R) ) 1 and 130 W, because of the higher outlet H2/CO ratio, than that at R ) 1.5 at 110 W, when both conditions were almost the same methane conversion and energy utility rate.26 However, the toxic PAHs emitted from the CH4/O2 rf plasma conversion process; therefore, the optimal operating conditions should be carried out at not only the higher conversion, syngas yield, syngas selectivity, and energy efficiency, but also the minimum carcinogenic PAH emission rate. In this study, because fewer toxic PAHs emitted can be achieved operating the rf plasma at a higher R, the better operating conditions should be modified and supported at R ) 1.5 and 110 W.
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Received for review February 1, 2005 Revised manuscript received June 22, 2005 Accepted June 24, 2005 IE050124Q
