Comparative Study on Polycyclic Aromatic Hydrocarbons, Light

Energy & Fuels 2003, 17, 999-1013

999

Comparative Study on Polycyclic Aromatic Hydrocarbons, Light Hydrocarbons, Carbon Monoxide, and Particulate Emissions from the Combustion of Polyethylene, Polystyrene, and Poly(vinyl chloride) Zhenlei Wang,† Jun Wang,† Henning Richter,‡ Jack B. Howard,‡ Joel Carlson,§ and Yiannis A. Levendis*,† Northeastern University, Boston, Massachusetts 02115, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and U.S. Army SBCCOMsNatick Soldier Center, Natick, Massachusetts 01760 Received November 19, 2002. Revised Manuscript Received May 15, 2003

A laboratory-scale study was performed to compare the emissions of pollutants from the batch combustion of polystyrene (PS), polyethylene (PE), and poly(vinyl chloride) (PVC) and to examine the conditions that minimize them. Fixed beds of polymer particles were burned in a two-stage, preheated muffle furnace, using air at atmospheric pressure. The temperature of the primary furnace was varied over a range of 500-1000 °C, to identify its influence on the emission of pollutants. The combustion effluent was mixed with additional preheated air, channeled to a secondary muffle furnace (afterburner), which was operated at 1000 °C. Emissions of CO, CO2, light hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and particulates were monitored either on-line or by sample collection, followed by gas chromatographic analysis. Emission magnitudes and trends were found to be dependent on the type of the polymer burned. The combustion of PS produced the largest yields of PAHs and, especially, soot; PE produced the next-largest yields, followed by those from PVC. Emissions of CO were the highest from PE. As the temperature of the primary furnace increased, (a) particulate emissions from burning PS and PVC generally increased, while the trend of PE was ambivalent; (b) PAH emissions from PS and PE increased, whereas those from PVC rather decreased; and (c) the trends of CO with temperature were not monotonic. Additional treatment of the combustion effluent in the afterburner led to a decrease of particulate and PAH emissions from PS and PVC, whereas those from PE decreased at low primary furnace temperatures but increased at high temperatures. CO emissions from PVC decreased in the afterburner, but those from PS and PE generally increased. The trends of emissions monitored at the exit of the afterburner paralleled those at the exit of the primary furnace, as the temperature of the primary furnace was varied. Given the aforementioned trends, it appears that operation of the primary furnace at a relatively low temperature, in the vicinity of 600 °C, allows the afterburner to operate under conditions suitable for minimizing most emissions for all three polymers. A detailed kinetic model, comprised of more than 1100 chemical reactions, was used for the description of the afterburner. For this purpose, the afterburner was approximated as a steady-state plug-flow reactor. Measured concentrations of O2, CO, CO2, light hydrocarbons, and PAHs at the inlet of the afterburner were integrated over the duration of the combustion event and used as input to the model calculation. Oxidation of PAHs, their conversion to soot, and the oxidation of soot were added to a previously developed model that included PAH formation. The model was tested for the case of PE incineration, and its predictions were qualitatively consistent with experimental data.

1. Introduction 1.1. The Waste Plastics Issue. Approximately 15 and 20 million tons of postconsumer plastic waste are generated annually throughout Europe and the United States, respectively.1,2 Polystyrene (PS), polyethylene (PE), and poly(vinyl chloride) (PVC) constitute a sig* Author to whom correspondence should be addressed. E-mail: [email protected]. † Northeastern University. ‡ Massachusetts Institute of Technology. § U.S. Army SBCCOMsNatick Soldier Center. (1) Williams, P. T. Waste Treatment and Disposal; John Wiley and Sons: Chichester, U.K., 1998.

nificant portion of municipal solid waste (7 wt % or 25 vol %; numbers vary with country and municipality). PE is the most abundant waste plastic (50-60 wt %), followed by PS (16-20 wt %). Polypropylene (PP) is the third-most-abundant waste plastic (10-16 wt %); however, it was not included in the present study, because its combustion and emissions bear pronounced similarities with those of PE.3,4 PVC is produced in compara(2) Rashid Khan, M.; Gorsuch, C. A. In Conversion and Utilisation of Waste Materials; Rashid Khan, M., Ed.; Taylor and Francis Publications: Washington, DC, 1996. (3) Panagiotou, T.; Levendis, Y. A. J. Appl. Polym. Sci. 1991, 43, 1549-1558.

10.1021/ef020269z CCC: $25.00 © 2003 American Chemical Society Published on Web 06/26/2003

1000

Energy & Fuels, Vol. 17, No. 4, 2003

tively smaller quantities (5-6 wt %) but has attracted significant attention, because of the potentially hazardous effect of chlorinated combustion byproducts. Disposal of waste plastics is considered to be a major environmental issue, because they are nonbiodegradable. Recently, the scarcity of space available for landfills and increased environmental concerns have resulted in an effort to abate landfill disposal of plastic wastes by means of increased recycling or energy recovery. In the recycling case, there are practical difficulties in the collection of recyclables, in their separation and decontamination, as well as in finding markets for them. In the case of energy recovery, although the thermal destruction of wastes is an effective method to reduce their volume, hazardous byproducts are generated and emitted into the atmosphere. Harmful health effects are likely to be associated with polycyclic aromatic hydrocarbons (PAHs), some of which have been found to be potentially toxic, carcinogenic, or mutagenic.5 PAHs also are thought to be precursors to soot formation.6 In this work, cumulative and individual PAHs, as well as soot produced in the combustion of the three major waste polymers (PS, PE, and PVC), were monitored. 1.2. Thermal Decomposition of Polymers. The combustion of solid polymers involves a complex sequence of the physical/chemical decomposition of material, with chemical reactions taking place in both the gas phase and the condensed phase, typically leading to a diffusion flame. The thermal degradation mechanism for PS has been shown to begin with chain scission, followed by additional random scission steps.7 As a result, the styrene monomer, together with the styrene dimer and trimer, has been previously identified as the major product of thermal degradation. Other degradation products have included benzene, toluene, ethylbenzene and methylbenzene, xylenes, indene, indane, and PAHs (such as naphthalene, fluorene, phenanthrene, and their alkylated derivatives).7-10 Small yields of light hydrocarbons were detected; these hydrocarbons consisted mainly of ethene, propene, butene, and methane.10 Regarding the pyrolysis of PE, a mechanism that involved the formation of free radicals via hydrogen abstraction was proposed by Rice and Rice.11 Simha et al.12 reported a similar general scheme to explain the thermal degradation of polymers. This mechanism involved several steps: initiation, propagation or freeradical transfer, and termination. Consistent with this mechanism, primary PE pyrolysis results in the formation of chains of various sizes with very few monomer units.10 Subsequently, secondary pyrolysis leads to (4) Panagiotou, T.; Levendis, Y. A. Combust. Flame 1994, 99, 5374. (5) Klaasen, C. D.; Amdur, M. O.; Doull, J. Casaret and Doull’s Toxicology, 2nd Ed.; Macmillan: New York, 1986; pp 84-139. (6) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26, 565-608. (7) Turi, E. Thermal Analysis; Brandrup and Immergut: Berlin, 1966. (8) Williams, P. T.; Horne, P. A.; Taylor, D. T. J. Anal. Appl. Pyrolysis 1993, 25, 325-334. (9) Audisio, G.; Bertini, F. J. Anal. Appl. Pyrolysis 1992, 24, 6174. (10) Williams, P. T.; Williams, E. A. Energy Fuels 1999, 13, 188196. (11) Rice, F. O.; Rice, K. K. The Aliphatic Free Radicals; John Hopkins Press: Baltimore, MD, 1936. (12) Simha, R.; Wall, L. A.; Bram, J. J. Chem. Phys. 1958, 29, 894.

Wang et al.

significant quantities of ethene (ethylene). For instance, the pyrolysis of PE in a fixed-bed reactor at a rate of 25 °C/min to a final temperature of 700 °C produced mainly ethene, propene, and butene.10 These alkenes are the product of the random scission process when coupled with stabilization of the resulting radical by hydrogen loss, leading to the formation of carbon double bonds. Also, alkanes such as methane, ethane, propane, and butane, which were formed as part of the random scission process of the thermal degradation of PE, were present in significant concentrations. Analysis of the oil/ wax of PE showed the presence of an exclusively aliphatic product that consisted of alkanes, alkenes, and alkadienes. No aromatic compounds were detected.10 Conesa et al.13 investigated the influence of temperature and residence time on the yields of products of highdensity polyethylene (HDPE) in a fluidized bed reactor. Experiments were conducted in the range of 500-900 °C. The yields of 13 pyrolysis products (methane, ethane, ethene, propane, propene, acetylene, butane, butene, pentane, benzene, toluene, xylenes, and styrene) were determined in each experiment. The results showed that the yield of total gas obtained increased as the temperature increased and reached a maximum yield at 800 °C; thereafter, it decreased. Scott et al.14 studied the fast pyrolysis of PE in an atmospheric-pressure fluidized-bed reactor under different operating conditions, including the use of activated carbon, in the temperature range of 515-790 °C. At temperatures 600 °C. These results illustrate that operation of the primary furnace at a temperature of 600 °C, in conjunction with the afterburner, seems to be a favorable condition for reducing the final particulate emissions of all three polymers. 3.4. Yields of Polycyclic Aromatic Hydrocarbons and Other Semivolatile Compounds. In this work, the PAHs detected in the condensed phase (filter extracts) and in the gas phase (XAD-4 extracts) were combined; thus, every PAH species presented herein is the sum of both phases. This was done to minimize uncertainty, because the exact distribution of various species in the two phases is known to be dependent on the temperature of the sampling stage (see ref 26 and references therein). Overall, the cumulative PAH emissions from PS and PE (the summation of all detected PAH species, both in the gaseous and condensed phases, not including the styrene oligomers) increased as the gas temperature of the primary furnace increased (see Figure 2e). PAH emissions from the combustion of PS were reduced by the afterburner (typically by a factor of 2), which was kept at a constant temperature of 1000 °C. Thus, the effect of the afterburner was beneficial in minimizing the final emissions of cumulative PAHs from PS. These results are consistent with those of Wang et al.26 on Styrofoam combustion. However, although the overall trend is remarkably similar, the detected magnitudes were higher in the present study, by as much as a factor of 2. This phenomenon might be related to higher concentrations of unpolymerized monomeric styrene in the fuel used in the present work. This is further supported by comparison with the PAH yields obtained by Westblad et al.33 from combustion of the styrene monomer. Under identical furnace conditions, the styrene monomer produced PAH yields that were greater than those of this study, typically by factors of 1.5-7. For PE, at 500 and 600 °C, the PAH emissions from the secondary furnace were lower than those at the exit of the primary furnace. However, at >600 °C, the cumulative PAH emissions from the secondary furnace were higher than those at the exit of the primary furnace. This is consistent with the similar temperature

Wang et al.

Figure 3. Plots of the yields of (a) the styrene monomer; (b) the styrene dimer and (c) the styrene trimer (all in mg/g), as a function of the primary furnace temperature. In all cases, the temperature in the afterburner was 1000 °C.

dependence of particulates emissions, as mentioned in the preceding section. However, the cross-over point, beyond which emissions from the secondary furnace exceeded those from the primary one, was observed at a higher temperature for particulate than for PAH emissions. These findings are consistent with the notion that PAHs contribute to particulate (soot) formation.6 As in the case of particulate emissions, the afterburner also minimized the final emissions of cumulative PAHs from PE at 500 and 600 °C; at higher temperatures, PAHs were generated therein. The PAH emissions from PVC combustion were low, and they steadily decreased as the temperature of the primary furnace increased. They were reduced by treatment in the afterburner. The yields of styrene oligomers released during combustion of PS are presented in Figure 3. At 500 °C, a large amount of styrene monomer was obtained at the exit of the primary furnace, totaling 100 mg/g (see Figure 3a). Given that only half of the primary furnace effluent was channeled to the first sampling stage, the total emission yield of styrene at this temperature was at least 200 mg/g, i.e., 20% of the sample mass. When the liquid styrene monomer was burned in previous

Emissions from the Combustion of PE, PS, and PVC

work under the same furnace conditions,33 the yield of unburned styrene was equivalent to 25% of the sample mass, which is comparable to that found herein. This indicates that a major portion of the polymer decomposes to the monomer under these low-temperature oxidative-pyrolysis conditions. The styrene yields decreased as the primary furnace temperatures increased. The secondary furnace effectively depleted the styrene monomer, either by conversion to PAHs or oxidation. At the lowest combustion temperature in the first stage (500 °C), large amounts of styrene dimer and trimer were also detected at the exit of the primary furnace, as shown in Figure 3b and c; their combined yield was 92 mg/g of fuel. In fact, these styrene oligomers were detected in the condensed phase (on the filter, not in the XAD) and were major components of the opaquewhite combustion-generated aerosol. The concentrations of styrene oligomers in the effluent of the primary furnace diminished at higher temperatures. In all cases, these species were completely depleted in the afterburner at 1000 °C. 3.4.1. Individual Polycyclic Aromatic Hydrocarbons at the Exit of the Primary Furnace. Profiles of 16 major PAH component yields (in units of micrograms of component per gram of sample) for PS, PE, and PVC are shown in Figure 4. PS and PE resulted in individual PAH yields that were higher than those of PVC at the exit of the first stage. In most cases, emissions from PS were higher than those from PE. In the case of both PS and PE, emissions were very low at a primary furnace temperature of 500 °C and increased as temperature increased thereafter. For most PAHs that were generated from PVC combustion, a decreasing trend was observed with increasing temperature. At all temperatures, the following was observed at the exit of the primary furnace: (a) the yields of naphthalene were the highest from all three polymers and were comparable for PS and PE; (b) acenaphthylene was the second-most-abundant PAH formed from PE; (c) phenanthrene was the second-most-abundant PAH formed from PS; (d) the yields of biphenyl, fluorene, anthracene, fluoranthene, acephenanthrylene, chrysene, benzo[g,h,i]fluoranthene, benzo[b]fluoranthene, and perylene from PS were higher than those from PE and were much higher than those from PVC; and (e) the yields of acenaphthylene, pyrene, cylopenta[c,d]pyrene, and benzo[a]pyrene from PS were often less than those from PE but were still much greater than those from PVC. The order of abundance of PAHs from the combustion of the three polymers is as follows (see Figure 4): (a) For PS: naphthalene; phenanthrene; biphenyl; acenaphthylene, fluorene, and fluoranthene; benzo[b]fluoranthene; anthracene, pyrene, and acephenanthrylene; chrysene; cyclopenta[c,d]pyrene; benzo[a]pyrene and benzo[g,h,i]fluoranthene; benzo[g,h,i]perylene; perylene. (b) For PE: naphthalene; acenaphthylene; phenanthrene; pyrene; fluoranthene; fluorene; biphenyl and anthracene; cyclopenta[c,d]pyrene, benzo[a]pyrene, acephenanthrylene, and benzo[b]fluoranthene; chrysene, benzo[g,h,i]fluoranthene, and benzo[g,h,i]perylene; perylene. (c) For PVC: naphthalene; phenanthrene; biphenyl and fluorene; acenaphthylene; fluoranthene; anthracene,

Energy & Fuels, Vol. 17, No. 4, 2003 1007

pyrene, acephenanthrylene, chrysene, and benzo[b]fluoranthene; benzo[g,h,i]fluoranthene; perylene; benzo[a]pyrene; cyclopenta[c,d]pyrene; benzo[g,h,i]perylene. 3.4.2. Individual Polycyclic Aromatic Hydrocarbons at the Exit of the Secondary Furnace. The effect of the secondary furnace (afterburner), which is always kept at a temperature of 1000 °C, was not uniform for all cases examined. The effect was dependent on both the polymer type and the temperature of the primary furnace under which oxidative pyrolysis took place. The trends observed for individual PAHs were as follows (see Figure 4): (a) For PS: Emissions of naphthalene, biphenyl, phenanthrene, fluorene, anthracene, acephenanthrylene, and chrysene were reduced by the afterburner at all oxidative pyrolysis temperatures. Emissions of acenaphthylene, fluoranthene, pyrene, cyclopenta[c,d]pyrene, benzo[a]pyrene, perylene, benzo[g,h,i]fluoranthene, benzo[b]fluoranthene, and benzo[g,h,i]perylene were reduced consistently when the primary furnace was operated at 500 and 600 °C. At primary furnace temperatures of 700-900 °C, these emissions increased, because of the afterburner treatment. For some of these compounds, the afterburner treatment reduced their emission again when the primary furnace was operated at 1000 °C. This complex behavior confirmed the observation made by Wang et al.26 on the emissions of combustion of commercial Styrofoam in the same apparatus. (b) For PE: All PAH compounds exhibited the same trend, inasmuch that their yields were reduced by the afterburner when the primary furnace was operated at 500 and 600 °C, whereas their yields increased at all higher primary furnace temperatures. (c) For PVC: With exception of naphthalene, for which almost no effect was observed, the afterburner was very effective in minimizing the emissions of all other PAH compounds at almost all primary furnace temperatures. Minor exceptions are the less-pronounced reductions of acenaphthylene, fluoranthene, and pyrene concentrations at a primary furnace temperature of 600 °C and of acenaphthylene at 500 °C. 3.5. Yields of Light Hydrocarbons. Light hydrocarbon emissions from the combustion of PS, PE, and PVC are comprised of mainly methane, ethane, ethylene, acetylene, propane, propylene, and butane. Traces of other compounds were detected by GC-FID; however, they were not identified in this study. Profiles of the five major light hydrocarbon species are shown in Figure 5. PE produced the largest yields for all five light hydrocarbons in the primary furnace, under all the investigated conditions. In the case of PE, in addition to the five light hydrocarbons shown in Figure 5, propane was identified at 600 °C, whereas butane was found at 800 °C. Ethylene was the most abundant light hydrocarbon species. The mole fractions of methane, ethane, and ethylene experienced maxima near 700 °C, which was in good agreement with the data of Scott et al.14 Above 700 °C, the yields of methane, ethane, and ethylene decreased. The acetylene yield peaked at 600 °C and declined thereafter as the temperature increased. The yield of propylene decreased as the temperature increased until a minimum was reached at 800 °C. The secondary furnace depleted all these compounds.

1008


Wang et al.



Figure 4. Plots of the yields of individual PAH components (µg/g) at the exits of the primary and secondary furnaces, as a function of the primary furnace temperature. In all cases, the temperature in the afterburner was 1000 °C.

The yields of the aforementioned five light hydrocarbon species from the combustion of PVC are also shown in Figure 5. In addition, propane and butane

were detected at 500 °C. The concentrations of all these species in the primary furnace increased and then decreased as the temperature rose from 500 °C to 1000

1010


Wang et al.

Figure 5. Plots of the yields of the light hydrocarbons methane, acetylene, ethylene, ethane, and propylene (all in µg/g) at the exits of the primary and secondary furnaces, as a function of the primary furnace temperature. In all cases, the temperature in the afterburner was 1000 °C.

°C. The maximum yield was achieved at 700 °C for methane and ethylene, whereas, at 600 °C, the maximum yields for ethane and propylene were obtained. Acetylene reached maximum yield at 800 °C. Similar to the case of PE, the secondary furnace depleted the yields of all these species. The light hydrocarbon emissions from PS are also shown in Figure 5. Emissions from the primary furnace

were mainly acetylene; however, methane, ethylene, and propylene also were present, in smaller amounts. The temperature dependence was less pronounced than in the case of the emissions from the other two polymers, with the exception of acetylene, which exhibited a local peak at 600 °C. Unlike the aforementioned cases, however, the secondary furnace had an ambivalent influence. The yields of methane and acetylene were


increased by the afterburner treatment at temperatures of >600 °C. This phenomenon might be related to significant amounts of styrene monomers that remain at the entrance of the afterburner, as shown in Figure 3a. These monomers are expected to decompose to C2 species (ethylene, vinyl), likely acetylene precursors, and single-ring aromatics, such as phenyl and benzene. The latter compounds, which are subsequently oxidized to phenoxy and phenol, can decompose to five-memberedring species (i.e., cyclopentadiene and its radical) and then to C2 and C3 species, which are possible precursors of methane and acetylene.32 At the lowest primary furnace temperature, no light hydrocarbon species were observed in the afterburner, despite high styrene monomer concentrations. In this case, very fuel-lean conditions prevailed in the afterburner, as indicated by continuous oxygen excess (see Figure 2a), allowing for an almost-quantitative oxidation of the light hydrocarbons. 3.6. Experimental Uncertainty. To test the reproducibility of the experiments, at least three (and up to five) runs were performed under the same operating conditions. In most cases, the agreement between the major gaseous emissions, individual PAH species, particulate emissions, etc. was within 10%. The worst cases observed in this study experienced disagreements on the order of (30%, relative to the arithmetic means of the values. Errors may arise from variabilities in the combustion technique itself (i.e., packing of the fuel beds, manual sample insertion, ignition location, flame dynamics). Small losses and variabilities may subsequently affect sampling, transfer, and the storage of filters and adsorbents. In addition, small uncertainties may be expected in the analytical techniques. 3.7. Kinetic Description of the Afterburner Treatment of Polyethylene Incineration. The afterburner treatment of the incineration of PE was described by means of detailed kinetic modeling. As in our previous work,26,33,34 the afterburner was approximated as a plugflow reactor, which allowed us to use the Senkin and Plug codes, which both are parts of the Chemkin software package.39 As described previously, O2, CO, CO2, light hydrocarbons, PAHs, and particulates were monitored at the inlet and the exit of the afterburner. Integration over the duration of the combustion event allowed for comparison with model predictions (see ref 26). The present work addressed previously identified shortcomings of the kinetic model, particularly the absence of pathways to describe the conversion of PAHs to soot, as well as their oxidation. At the current stage, the model consists of 277 species and 1121 reactions. The oxidation of major aliphatic and aromatic hydrocarbons and the formation of PAHs that contain up to seven condensed aromatic rings are described by means of elementary reactions. The pressure dependence of chemically activated reactions has been taken into account. Prior to the additions made in the present work, the model had been exhaustively tested in premixed low-pressure flames28,29,32 and a well-stirred(39) Kee, R. J.; Rupley, F. M.; Miller, J. A.; Coltrin, M. E.; Grcar, J. F.; Meeks, E.; Moffat, H. K.; Lutz, A. E.; Dixon-Lewis, G.; Smooke, M. D.; Warnatz, J.; Evans, G. H.; Larson, R. S.; Mitchell, R. E.; Petzold, L. R.; Reynolds, W. C.; Caracotsios, M.; Stewart, W. E.; Glarborg, P.; Wang, C.; Adigun, O. CHEMKIN collection, release 3.6; Reaction Design, Inc.: San Diego, CA, 2000.


coupled-to-plug-flow-reactor setup at atmospheric pressure.31 In the present work, global kinetic data that described the oxidation of PAHs (except naphthalene, for which elementary reactions have been used28,29), as well as their conversion to soot, were added to the reaction mechanism. The experimental determination of these kinetic data has been described in detail in a recent publication35 and is outlined only briefly in the present paper. In the work of Wang et al.,35 pure oxygen, instead of air, was added at the venturi, which led to the total suppression of soot formation as well as PAH growth in the afterburner. The consumption of individual PAHs within the afterburner (and those formed in the primary furnace prior to oxygen addition at the venturi) was used for the deduction of overall bimolecular rate constants, kox, to describe the oxidation, by O2, of individual PAHs. For most PAHs, the value of kox increased from ∼1 × 104 cm3 mol-1 s-1 to 1 × 106 cm3 mol-1 s-1 in the investigated temperature range of 900-1100 °C. Subsequently, the global rate constants that describe the conversion of individual PAHs to soot, ksoot, were determined in a different set of experiments that were conducted under sooting conditions in which simultaneous PAH oxidation has been taken into account and was described using the previously determined kinetic data. The value of ksoot was found to be in the range of ∼1 × 108-1 × 1010 cm3 mol-1 s-1.35 In addition to the description of soot formation, soot oxidation must be included in kinetic models, to allow for assessment of the competition between the formation and consumption pathways. Therefore, a rate constant to assess soot oxidation by O2 has been approximated to be 1 × 10-6 g cm-2 s-1, based on data reported previously (see Figure 4 in ref 34). This value, in conjunction with an approximate soot surface area of 200 m2 g-1 and a partial oxygen pressure of 0.146 atm,34 resulted in a bimolecular rate constant that corresponded to the oxidation of soot by reaction with O2 (ksootox ) 1.4 × 106 cm3 mol-1 s-1). At the current stage, and because of the small OH concentrations in the afterburner, in comparison to those of O2, under the conditions investigated here, soot oxidation via reaction with OH40 has not been included in the reaction network. In the present work, the afterburner temperature always has been kept constant, at 1000 °C; therefore, constant values were used for the kinetic data that were added to the reaction mechanism: kox ) 1 × 105 cm3 mol-1 s-1, ksoot ) 1 × 109 cm3 mol-1 s-1, and ksootox ) 1.4 × 106 cm3 mol-1 s-1. Note that the kinetic parameters used here are not intended to describe elementary reaction steps. The oxidation of large PAHs, as well as the formation and oxidation of soot, are processes of extreme complexity and the use of global kinetic data represents an approximation of sequences with many individual reactions. However, the addition of such global rate constants is a first major step toward a complete description of the entire combustion process, including PAH- and soot-forming conditions. All the previously discussed global rate parameters have been determined using PS combustion; therefore, (40) Neoh, K. G.; Howard, J. B.; Sarofim, A. F. In Particulate Carbon Formation During Combustion; Siegla, D. C., Smith, G. W., Eds.; Plenum Publishing Corporation: New York, 1981; pp 261-282.

1012


Wang et al.

Figure 6. Comparison of model predictions for naphthalene, phenanthrene, and benz[a]pyrene with experimental data at the entrance and exit of the afterburner. The temperature of the primary furnace was 600 °C, and the temperature in the afterburner was 1000 °C. Legend is as follows: (b) naphthalene, experimental data; (s) naphthalene, prediction; (9) phenanthrene, experimental data; (- - -) phenanthrene, predicted; (2) benzo[a]pyrene, experimental data; and (- - -) benzo[a]pyrene, prediction.

their validity had to be tested for at least one other fuel. Thus, the model has been tested for two-stage PE combustion. Experiments at primary furnace temperatures of 600 and 900 °C were selected, while the afterburner temperature was kept constant at 1000 °C. Similar to the procedure described previously,26 integrated experimental concentrations of O2, CO, CO2, and 16 major PAHs were used as input for the plug-flow reactor calculations. Concentrations of H2 and H2O at the exit of the primary furnace were estimated by means of perfectly stirred reactor computations, using the Aurora code;39 dilution with O2 at the venturi was taken into account.26 Different from our previous work,26,33,34 light hydrocarbons were measured, as previously discussed, and integrated concentrations of CH4, C2H2, C2H4, and C3H6 were also used as input for the model calculations. The first question addressed was the assessment of the gas temperature in the afterburner. The wall furnace temperature was kept at 1000 °C; however, higher temperatures of the reacting mixture, because of exothermic processes, cannot be excluded and could have a significant impact on the combustion chemistry, including the formation and depletion of pollutants such as PAHs and soot. Temperature measurements in a reactive flow are complex, and significant errors cannot be excluded. To obtain the upper limit of the expected gas temperatures, the adiabatic temperature profile of the gaseous mixture in the afterburner was computed using the Plug code39 for the case of a primary furnace temperature of 600 °C and an afterburner inlet gas temperature of 1000 °C. A maximal increase of 95 K was determined, under the described adiabatic conditions, and is consistent with an even smaller increase of the gas temperature in the afterburner during the

combustion event found experimentally by means of thermocouple measurements. Therefore, all further computations were conducted assuming a constant afterburner gas temperature of 1000 °C. The comparison of model predictions with experimental data at the outlet of the afterburner showed the following picture. 3.7.1. Primary Furnace Temperature of 600 °C. In the case of a primary furnace temperature of 600 °C, the computed and experimental PAH concentrations were both reduced by the afterburner treatment. In Figure 6, predicted mole fraction profiles of selected PAHs of high abundance (naphthalene) or those which are suspected to have significant health hazardous effects (phenanthrene and benzo[a]pyrene) are compared with experimental data at the inlet and outlet of the afterburner. For most other PAHs and similar to phenanthrene, PAH consumption was less pronounced in the model predictions than that found experimentally. Analysis of the modeling results showed that both oxidation and conversion to soot contributed to PAH consumption, with oxidation being ∼3-6 times more important. Soot oxidation was found to be significant; however, different from the experimental data, where soot was depleted in the afterburner in the case of a primary furnace temperature of 600 °C (Figure 2d), its formation was predicted for both investigated conditions. Nevertheless, less soot was predicted to be formed for a primary furnace temperature of 600 °C than in the case of 900 °C, which is consistent with experimental data. 3.7.2. Primary Furnace Temperature of 900 °C. In the case of a primary furnace temperature of 900 °C, the model did not reproduce the experimentally observed PAH formation in the afterburner; however,


predicted PAH consumption was found to be significantly less pronounced than that for the 600 °C case. The conversion to soot, in this case, contributed more to PAH consumption than oxidation, despite the partial oxygen pressure being higher than that at 600 °C (see Figure 2a). Soot formation is underpredicted by a factor of ∼20, which can be, at least partially, attributed to the lack of computed PAH formation in the afterburner. 3.7.3 Conclusions on the Modeling Effort. On the basis of these results, it can be concluded that the updated kinetic model allows the prediction of trends that are consistent with experimental observations; however, the remaining quantitative discrepancies are significant. To assess the reasons for these discrepancies, the effect of potential uncertainties in the input parameters was tested. For instance, a 1000-fold increase of the methane input concentration had no visible effect on the predicted concentration of polycyclic aromatic hydrocarbons (PAHs), although, in agreement with the experimental findings (Figure 5), methane depletion in the afterburner was complete. Similarly, 10-fold increases of the ethylene and acetylene mole fractions, which changed independently, in the input of the plug-flow reactor calculations for the 900 °C case affected the predicted PAH and soot concentrations by e10%. Therefore, uncertainties in data that are used as input for the plug-flow reactor calculations are unlikely to be a major source of discrepancies between the model predictions and the experimental data. Thus, the existence of additional reaction pathways, currently not included in the model, cannot be excluded. For instance, the lack of PAH formation in the case of a primary furnace temperature of 900 °C might be related to additional growth reactions, such as direct acetylene attack to PAH-CHCH adducts followed by hydrogen loss.41,42 Also, the use of global rate constants for the description of the oxidation of PAHs and their conversion to soot, as well as soot oxidation, are likely to contribute to the discrepancies that are observed between model predictions and experimental data. Future work should include the extension of soot modeling to more-detailed chemistry, for instance, the use of a sectional approach, which is currently under development.43,44 Also, experiments under steady-state conditions are planned, to minimize the uncertainties that are related to the transient combustion and the resulting approximations that are necessary in the present work. Acknowledgment. The authors acknowledge Mr. Eric Wisnaskas for his assistance. This project was funded by the National Science Foundation (NSF), under Grant No. CTS-9908962 (Dr. Farley Fisher, program director). (41) Bittner, J. D.; Howard, J. B. Proc. Combust. Inst. 1981, 18, 1105-1116. (42) Kislov, V. V.; Mebel, A. M.; Lin, S. H. J. Phys. Chem. A 2002, 106, 6171-6182. (43) Pope, C. J.; Howard, J. B. Aerosol Sci. Technol. 1997, 27, 7394. (44) Richter, H.; Granata, S.; Howard, J. B.; Kronholm, D. F. Modeling of PAH and Soot Formation in a Laminar Premixed Benzene/ Oxygen/Argon Low-Pressure Flame. In Proceedings of the Third Joint Meeting of the U. S. Sections of the Combustion Institute, Chicago, March 2003.


Appendix: Calculation of Carbon Balances Carbon balances during the combustion of polymers were calculated at different temperatures, as illustrated in the examples below: A.1. Combustion of Polyethylene at 700 °C. One gram of input polyethylene (PE) contains 860 mg of carbon; this amount is balanced against the carbon contained in the products at the exits of both the primary and the secondary furnaces:

12 12 + mCO2 × + 28 44 stage1 12 12 mCO × + mCO2 × + (msoot)stage1 + 28 44 stage2 (msoot)stage2 + (mlight hydrocarbon)stage1,2

[(

[(

)]

) (

12 12 + 889.64 × + 28 44 stage1 12 12 333.23 × + 1140.00 × + 17.54 + 28 44 stage2 11.50 + 18.70 ) 789.94 mg

[(

)]

) (

) 106.98 ×

[(

)]

) (

mtotal ) mCO ×

)]

) (

Thus, the carbon is 92% balanced. A.2. Combustion of Polystyrene at 1000 °C. One gram of input polystyrene (PS) contains 920 mg of carbon; this amount is balanced against the carbon contained in the products at the exits of both the primary and the secondary furnaces:


[(

[(

)]

) (

mtotal ) mCO ×

)]

) (

12 12 + 777.34 × + 28 44 stage1 12 12 146.03 × + 849.66 × + 157.90 + 28 44 stage2 126.23 + 4.34 ) 846.44 mg

[(

[(

)]

) (

) 120.53 ×

)]

) (

Thus, the carbon is 92% balanced. A.3. Combustion of Poly(vinyl chloride) at 1000 °C. One gram of input poly(vinyl chloride) (PVC) contains 380 mg of carbon; this amount is balanced against the carbon contained in the products at the exits of both the primary and the secondary furnaces:


[(

mtotal ) mCO ×

[(

)]

) (

)]

) (

12 12 + 491.26 × + 28 44 stage1 12 12 18.06 × + 553.48 × + 28.14 + 6.00 + 28 44 stage2 0.35 ) 341.87 mg

[(

) 34.33 ×

[(

) (

)]

) (

)]

Thus, the carbon is 90% balanced. EF020269Z

Comparative Study on Polycyclic Aromatic Hydrocarbons, Light

Recommend Documents