Emissions from Premixed Combustion of Polystyrene - Energy & Fuels

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Energy & Fuels 2008, 22, 354–362

Emissions from Premixed Combustion of Polystyrene Cecilia K. Gonçalves,† Jorge A. S. Tenório,† Yiannis A. Levendis,* and Joel B. Carlson‡ Department of Mechanical and Industrial Engineering, Northeastern UniVersity, Boston, Massachusetts 02115 ReceiVed July 23, 2007. ReVised Manuscript ReceiVed NoVember 1, 2007

Pyrolysis of polystryrene (PS) followed by combustion was tested in a two-stage drop-tube furnace at steadystate steady-flow conditions. PS particles were pyrolyzed at 1000 °C in nitrogen, and subsequently, the pyrolyzate gases were mixed with oxygen-containing gases and were burned homogeneously at 900, 1000, or 1100 °C. Nominally premixed combustion occurred at different equivalence ratios, depending on the feed rates of the polymer. The effluents of pyrolysis and combustion were analyzed for fixed gases (CO, CO2, O2), light hydrocarbons, PAH, and particulates. It was found that the yields of pyrolysis gas decreased with increasing polymer feed rate. CO2 emissions peaked at an equivalence ratio near unity, while the CO emissions were significantly large only at fuel-rich equivalence ratios, φ > 1. The total light hydrocarbon and the PAH yields from combustion of the PS pyrolyzates increased with increasing equivalence ratio. The generated particulates were mostly in the ranges of 0–0.4 and 0.65–1.10 µm. Overall, PAH and soot emissions from the indirect burning of PS, i.e., the homogeneous combustion of the pyrolyzates, were an order of magnitude lower than corresponding emissions from direct burning of the solid polymer, as previously monitored in this laboratory using identical sampling and analytical techniques. Therefore, pyrolysis/gasification of PS followed by combustion is shown to be a credible method for low-emission waste-to-energy conversion.

1. Introduction The total amount of plastics ending up in the US Municipal Solid Waste (MSW) stream in 2005 was 28.9 million tons, of which only 1.65 million tons (5.7%) was recovered. Most of the rest was landfilled, with only a small fraction burned for energy recovery.1 This last option, however, can be advantageous as it reduces the volume of the MSW by around 90%,2 and the large amount of energy stored in the plastics (on the order of 40 MJ/kg) can be converted to heat or electricity. Previous work has attested to the very sooty direct combustion of solid polymers,3–5 as also evidenced in the photographic sequence of combustion of a commonly used polystyrene cup, shown in Figure 1. Such processes typically generate copious emissions of gaseous and particulate pollutants. As an alternative to direct combustion, high volume waste plastics can be converted through high-temperature gasification/ pyrolysis, to a large extent, into a blend of gaseous hydrocarbons. This gaseous fuel, thoroughly mixed with air, may be subsequently burned under premixed homogeneous conditions to minimize the emissions of products of incomplete combustion (PIC), which include gaseous and particulate pollutants. * Corresponding author: phone +1 617 373-3806; fax +1 617 373-2921; e-mail [email protected]. † Department of Metallurgical and Materials Engineering, University of São Paulo, Av. Professor Mello Moraes, 2463, 05508-900 São Paulo, SP, Brazil. ‡ US Army SBCCOM—Natick Soldier Center, Natick, MA 01760. (1) EPA—Environmental Protection Agency. Municipal Solid Waste in the United States: Facts and Figures for 2005, www.epa.gov. (2) Wang, Z.; Ritcher, H.; Howard, J. B.; Jordan, J.; Carlson, J.; Levendis, Y. A. Ind. Eng. Chem. Res. 2004, 43, 2873–2886. (3) Panagiotou, T.; Levendis, Y. A. Combust. Sci. Technol. 1998, 137, 121–147. (4) Shemwell, B.; Levendis, Y. A. J. Air Waste Manage. Assoc. 2000, 50, 94–102. (5) Wang, Z.; Wang, J.; Ritcher, H.; Howard, J. B.; Carlson, J.; Levendis, Y. A. Energy Fuels 2003, 17, 999–1013.

In terms of energy, the heat necessary to pyrolyze the PS (1.5 MJ/kg) is only a small fraction of the internal energy of the polymer itself (heating value of 40.2 MJ/kg).6,7 Thus, the energy balance is largely positive, and a very favorable heat integration may be achieved in the production of this wastepolymer-derived pyrolyzate fuel. Polystyrene (PS) melts at 237.5 °C and starts decomposing at 364 °C. Pyrolysis of PS in the temperature range of 532–708 °C has been shown to produce large amounts (75 wt %) of the styrene monomer precursor (whose boiling point is 146.2 °C) together with other aromatic and polycyclic aromatic hydrocarbons (∼10 wt % toluene, ethylbenzene, propenylbenzene, propynylbenzene, and naphthalene).8,9 Other works also pyrolyzed PS in the temperature range of 360–410 °C10 and at 520 °C11 and found mainly styrene, some dimers, and trimers. Darivakis et al.12 reported that the total volatile yield (total weight loss) from rapid devolatilization of PS increases with temperature and approaches 100% as the temperature of 800 °C is reached. If temperatures drop, some of this yield condenses as oils or tars, consisting mostly of styrene. Darivakis reported the condensable fraction as 30 wt %, whereas Scott et al.8 (6) Jinno, D.; Gupta, A. K.; Yoshikawa, K. Thermal Destruction of Surrogate Solid Waste. Proceedings of the 26th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, FL, March 2001. (7) Jinno, D.; Gupta, A. K.; Yoshikawa K. Thermal destruction of Plastic Materials in Solid Waste. Proceedings of the 27th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, FL, March 2002. (8) Scott, D. S.; Czernik, S. R.; Piskorz, J.; Radlein, A. G. Energy Fuels 1990, 4, 407–411. (9) Williams, P. T.; Williams, E. A. Energy Fuels 1999, 13, 188–196. (10) Bockhorn, H.; Hornung, A.; Hornung, U. Proc. Combust. Inst. 1998, 27, 1343–1349. (11) Kaminsky, W.; Predel, M.; Sadiki, A. Polym. Degrad. Stab. 2004, 85, 1045–1050. (12) Darivakis, G. S.; Howard, J. B.; Peters, W. A. Combust. Sci. Technol. 1990, 74, 267–281.

10.1021/ef700431f CCC: $40.75  2008 American Chemical Society Published on Web 12/21/2007

Emissions from Premixed Combustion of Polystyrene

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Figure 1. Photographic sequence of the smoky combustion of a commonly used polystyrene cup (Styrofoam).

reported much higher values, in the vicinity of 85%. Therefore, if the temperature of the pyrolyzate stream is maintained at sufficiently high levels (certainly above the boiling point of styrene), then a mostly gaseous stream of pyrolyzates may be delivered to the oxidizer furnace. Identification of individual PAH components in the pyrolysis products of PS particles in furnaces, heated at temperatures of 1000 and 1100 °C, has been performed by Panagiotou et al.13,14 Emissions from direct combustion of PS have been studied elsewhere and in this laboratory.15,16 Previous research in this laboratory addressed the emissions of inorganic compounds, volatile and semivolatile organic compounds, including PAH, and condensed-phase compounds from combustion of PS in batch-horizontal furnaces2,5,16,17 and drop-tube steady-state steady-flow reactors.3,4,13,14,18 Those studies evaluated the effects of sample mass (equivalence ratio), the furnace temperatures and residence times, single-stage and two-stage combustion, and the effects of high-temperature flue gas filtration, among other parameters. In recent work, Levendis and co-workers19–21 summarized and compared these results to the emissions from burning styrene in batch-horizontal furnace and from burning gasified ethylbenzene in a one-dimensional flat flame. The latter mode of combustion, being homogeneous and premixed, generated much lower PAH and particulate emissions than did direct nonpremixed combustion of PS, at comparable equivalence ratios. Such observations gave impetus to this work, i.e, to gasify the solid polymer, mix it with air, and then ignite the charge and burn it homogeneously at a preselected optimum equivalence ratio. This work, on indirect combustion of polystyrene (PS), complements concurrent work on indirect combustion of polyethylene (PE), which is described in a related publication.22 However, whereas PE can be pyrolyzed to a nearly quantitative stream of light volatile hydrocarbons which are most suitable for the production of a gaseous fuel, the pyrolyzates of PS may also be suitable to be used in the production of an oil/wax feedstock for the petrochemicals industry. Nevertheless, as separation of mixed plastics in a waste stream is not always easy, polyethylene, polystyrene, polypropylene, and other plastics may be gasified together to a gaseous fuel blend, which eventually can be burned for power generation. Hence, this work (13) Panagiotou, T.; Levendis, Y. A.; Carlson, J.; Dunayevskiy, Y. M.; Vouros, P. Combust. Sci. Technol. 1996, 1116–117, 91–128. (14) Panagiotou, T.; Levendis, Y. A.; Carlson, J.; Vouros, P. Proc. Combust. Inst. 1996, 26, 2142–2460. (15) Durlak, S. K.; Biswas, P.; Shi, J.; Bernhard, M. J. EnViron. Sci. Technol. 1998, 32, 2301–2307. (16) Wang, J.; Levendis, Y.; Ritcher, H.; Howard, J. B.; Carlson, J. EnViron. Sci. Technol. 2001, 35, 3541–3552. (17) Wang, J.; Ritcher, H.; Howard, J. B.; Levendis, Y.; Carlson, J. EnViron. Sci. Technol. 2002, 36, 797–808. (18) Courtemanche, B.; Levendis, Y. A. Fuel 1998, 77, 183–196. (19) Ergut, A.; Levendis, Y. A. Comparison of PIC of waste PS and styrene in diffusion flames and of ethylbenzene in fuel rich premixed flames. Proceedings of ASME POWER, Baltimore, MD, 2004. (20) Ergut, A.; Levendis, Y. A. Fuel 2007, 86, 1789–1799. (21) Westblad, C.; Levendis, Y.; Ritcher, H.; Howard, J.; Carlson, J. A. Chemosphere 2002, 49, 395–412. (22) Goncalves, C. K.; Tenorio, J. A. S. T.; Levendis, Y. A.; Carlson, J. B. Emissions from premixed combustion of gasified polyethylene. Energy Fuels, in press.

Figure 2. Schematic of the two-stage gasifier and oxidizer furnace with (a, left) the sampling stage and (b, right) the particle impactor stage, placed at the bottom of the furnace. Legend: 1, entrance of N2; 2, entrance of N2/O2 mixture; 3, light hydrocarbon collection point; 4, condensed PAH and particulate matter material (filter paper); 5, volatile PAH (XAD resin); 6, light hydrocarbon collection point; 7, online analyzers O2, CO2, and CO.

examines the emission reduction benefits from such a gasificationcombustion scheme. 2. Experimental Apparatus and Procedure PS in granular form was procured from Chemical Aldrich and was ground and sieved to 38–125 µm particles. Three sets of experiments were performed to assess the emissions of indirect combustion of PS: (i) Pyrolysis followed by combustion tests in a two-stage apparatus were coupled with light hydrocarbons (LH) only sampling from the pyrolysis (first stage) as well as with CO, CO2, LH, PAH, and cumulative particulate sampling from the combustion (second stage). (ii) Pyrolysis followed by combustion tests in a two-stage apparatus were coupled with LH only sampling from the pyrolysis (first stage) as well as particulate size distribution sampling from the combustion (second stage). (iii) Pyrolysis-only tests in a one-stage apparatus were conducted to enable simultaneous LH, PAH, and total particulate analysis. In the pyrolysis followed by combustion tests, case i above, indirect combustion was carried out in a two-stage electrically heated, drop-tube furnace, illustrated in Figure 2a and detailed elsewhere.22 A nitrogen flow of 2 L/min was introduced to the gasifier/pyrolyzer to fluidize the polymer. The injection rate varied from 0.2 to 1.2 g/min of polymer particles to obtain the targeted equivalence ratios from 0.3 to 1.75. The equivalence ratio is defined as follows: φ ) [(mfuel/mair)actual/(mfuel/mair)stoichiometric]. The first furnace was operated at 1000 °C. Upon pyrolysis of the fuel in the first furnace, the effluent went through a venturi, where it was mixed with 3.6 L/min total of oxygen-nitrogen gases, preheated to 300 °C. Afterwards, the mixture was conducted to the oxidizer furnace where combustion occurred. The composition of the nitrogen-oxygen gases entering the venturi was such that the resulting mole fraction of oxygen in the gas entering the oxidizer furnace was 21%. The oxidizer furnace was operated at 900, 1000, and 1100 °C to evaluate the effect of the gas temperature. Test durations were in the range of 2–6 min. Upon exiting the oxidizer furnace, the combustion effluent gas was channeled to CO and CO2 nondispersive infrared analyzers (Horiba) and to O2 paramagnetic analyzers (Beckman model 250). The analyzer signals were recorded with a data acquisition card (Data Translation model DT322) as mole fractions.

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Figure 3. Diameter of a 125 µm PS particle vs (a) time to gasify in milliseconds and (b) traveled distance to complete gasification in centimeters under the conditions of this work.

Sampling for the composition of the light hydrocarbons took place at the exits of both furnaces by withdrawing gases with microsyringes and analyzing their contents with GC-FID. The composition of the PAH components was assessed with a sampling stage, encompassing a filter paper to capture condensed-phase PAH as well as the total particulate matter, and a chamber containing 30 mL of XAD-4 resin to capture gas-phase PAH. The sample conditioning and extraction techniques along with the analytical techniques for detection of individual PAH species, by gas chromatography coupled to mass spectrometry (GC-MS), were explained in detail in previous works.14,17 In these tests, 23 light hydrocarbon species and 53 PH species were identified and quantified. The most prevalent of those species are presented herein. In all tests, the consistency of the element mass balance (carbon) between the inlet and the outlet of the gasifier/pyrolyzer and the oxidizer furnaces was checked. To measure the size distribution of the combustion-generated particulates (see case ii above), the aforementioned sampling stage below the furnace was replaced with a multistage Andersen particle impactor (see Figure 2b). This instrument is composed of a series of perforated disks (stages) capable of separating the particles in nine groups, starting with particle size bigger than 9 µm down to 0.4 µm. At the last stage a filter paper collected particles smaller than 0.4 µm. In these particular tests the gasifier furnace was operated at 1000 °C and the oxidizer furnace was operated at 1100 °C. The test durations were 4.5–12 min. The aforementioned sampling stage to collect PAH species was rather bulky and could not be inserted between the furnaces to sample the pyrolysis effluent of the first stage. Hence, to enable these tests, one of the furnaces was removed. A limited series of such pyrolysis only tests (case iii above) were performed using a flow of 2 L/min of nitrogen gas and a particle injection rate in the range of 0.57–0.62 g/min. These injection rates correspond to calculated equivalence ratios in the range of 0.40–0.43. The furnace was again operated at 1000 °C; light hydrocarbons, PAH, and total particulate were sampled.

3. Results 3.1. Particle Gasification Calculation/Simulation. The available length of the upper furnace for a particle to gasify is 19.5 cm, which includes the heating zone of the furnace of 11 cm. To investigate whether such a furnace length was sufficient for PS particles to gasify, calculations and numerical computations were performed to determine both the residence time and the trajectory length of the largest size PS particles used herein, i.e., 125 µm. The gasification temperature of PS particles was assumed to be 500 °C. The temperature profile of the furnace, the inputs, and methods of the calculation were described by Goncalves et al.22 The residence time of the N2 gas flowing through the gasifier furnace was determined to be 1.6 and 1.56 s by the numerical computation and the calculation, respectively. The heat-up and gasification times and distance traveled for a PS polymer particle in the gasifier furnace calculated from both methods are shown in Figure 3. The time to gasify the PS was found to be 241 and 152 ms in calculation and numerical computation, respectively.

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Figure 4. Mole fractions of the main quantified (a) light hydrocarbons (LH) and (b) polycyclic aromatic hydrocarbons (PAH) in the pyrolysis gas.

The distance traveled of the solid particle was found to be very similar with both methods, i.e., 9.6 and 9.33 cm, in calculation and numerical computation, respectively. These values are half the aforementioned available length of the furnace. Even if an error factor of 2 is applied on the distance encountered in the numerical computation, PS particles (38–125 µm) are still expected to gasify within the hot zone of the furnace. 3.2. Yields from Pyrolysis at 1000 °C. Light Hydrocarbons Yields (LH). The relative amounts of eight predominant LH components in the pyrolysis gas, representing nearly 100% of the total measured light hydrocarbons, are shown in Figure 4a. The total and individual light hydrocarbon yields are shown in Figure 5. It should be mentioned here that the light hydrocarbon component styrene was not quantified reliably in this work. As the feed rate increased in the range of 0.2–1.2 g/min, corresponding to nominal equivalence ratios in the subsequent oxidizer furnace in the range of 0.3–1.75, the yields of these gaseous species decreased slightly. PAH Yields. The relative amounts of the 14 prevalent components in the pyrolysis gas, representing almost 80% of the total PAH, are shown in Figure 4b. Such compounds were also reported in the literature, as major PAH products from pyrolysis of PS at high temperatures.14 The total and individual PAH yields are shown in Figure 6. The PAH yields present in pyrolysis effluents were only examined in a narrow range of feed rates (0.57–0.62 g/min) and were related to the equivalence ratio (0.4–0.43) in the oxidizer furnace. The major PAH products of pyrolysis were phenanthrene, naphthalene, biphenyl, acenapthylene, anthracene, indene, benz[a]anthracene, fluorene, fluoranthene, and pyrene. 3.3. Yields from Pyrolysis Followed by Combustion at 900, 1000, and 1100 °C. Oxygen, Carbon Monoxide, and Carbon Dioxide Emission Yields. The combustion of PS pyrolyzates (generated at 1000 °C) in the second stage furnace resulted in oxygen and carbon oxide profiles similar to those shown in Figure 7. Fluctuations occur due to particle flow instabilities in the fluidizing system. Results shown in Figures 8 and 11 correspond to average values in each test. The O2, CO, and CO2 mole fractions present in the oxidizer furnace effluent upon combustion the of the PS pyrolyzates (at 900, 1000, and 1100 °C) are presented in Figure 8 and are compared to theoretically calculated values based on the different φs. CO and CO2 yields are shown in Figure 11 along with comparisons with previous results from direct combustion of the same polymer. The O2 mole fraction decreased from 20 to 1% as the equivalence ratio increased from 0.3 to 1.75. In accordance with theoretical values, the average CO2 mole fractions peaked (∼12%) at the equivalence ratio of unity. At 1100 °C, a slightly higher production of CO2 and an associated higher consumption of O2 were observed, as compared to those at lower temperatures, indicative of better combustion. In agreement with the theoretical predictions, the CO formation at equivalence ratios



Figure 5. Total light hydrocarbons yields and prominent individual light hydrocarbon yields (mg/g) upon PS pyrolysis at 1000 °C and combustion at 900, 1000, and 1100 °C as functions of the equivalence ratio.

lower than 1 was negligible, indicative of effective mixing of pyrolyzate gases with the N2/O2 gases, resulting in a nominally premixed flame. The CO emissions became significant above φ ) 1, as oxygen became scarce. Light Hydrocarbons Yields. Increasing equivalence ratios increased the total unburned light volatile hydrocarbons yields from combustion of PS, as shown in Figure 5. Methane, acetylene, and ethylene emissions exhibited similar behaviors, i.e., their output yields of combustion exceeded the input to the oxidizer furnace pyrolysis yields, under fuel-rich conditions. This provides evidence of formation of these compounds in the oxidizer furnace. Propylene, 1,3-butadiene, benzene, toluene, and styrene yields after combustion remained below the levels of the corresponding pyrolysis yields, even at the highest φs studied. This suggests that these compounds are pyrolysis products that are consumed during the oxidation stage, even though their formation during this stage is not precluded. The input to the oxidizer furnace pyrolysis yields of ethylacetylene, benzaldehyde, and phenol were comparable to the output yields therefrom. There was no general trend in the yields of the individual compounds with oxidizer furnace temperature. For instance, propylene was not detected at 900 °C, and only small amounts of this compound were detected at the higher φs at 1000 and 1100 °C, suggesting its destruction at the lowest temperature. However, 1,3-butadiene was not detected at 1100 °C, and it was only detected

at 900 °C at φ ) 0.5, whereas at 1000 °C it was only detected at φ ) 1.6–1.7, indicating destruction at the highest temperature of 1100 °C. PAH Yields. Upon combustion, the total PAH yields were reduced from their pyrolysis levels. The latter, however, were only measured in a narrow fuel-lean region; this was also the case for most individual PAH compounds. Total PAH amounted to as much as 10 mg/g of polymer injected under fuel-rich conditions. Yields increased with increasing equivalence ratios in most cases (see Figure 6). Similar to the pyrolysis effluent, phenanthrene and naphthalene were again the most pronounced compounds in the combustion effluents, followed by acenapthylene. The effect of the oxidizer furnace temperature, in the examined range of 900–1100 °C, on the PAH emissions is not clear. This contrasts to the findings of Wheatley et al.,24 who reported that increasing furnace temperatures, in a comparable range, undoubtedly reduced the PAH emissions from direct combustion of PS. Biphenylene and dibenzofuran were present in the pyrolysis gas in small amounts; however, they exhibited slightly higher yields in the combustion effluent than in the pyrolysis gas. This behavior suggests that the conditions in the oxidizer furnace (23) Pantalone, J. C.; Ergut, A.; Levendis, Y. A. Combust. Sci. Technol. 2006, 178, 1297–1324. (24) Wheatley, L.; Levendis, Y. A.; Vouros, P. EnViron. Sci. Technol. 1993, 27, 2885–2895.


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Figure 6. Total PAH yields and yields of individual PAH components (mg/g) upon pyrolysis at 1000 °C and combustion at 900, 1000, and 1100 °C as functions of the equivalence ratio.

Figure 7. Typical mole fraction-time profiles of fixed gases: CO, CO2, and O2. Tgasifier ) 1000 °C, Toxidizer ) 1100 °C, and φ ) 0.80.

were not sufficient to promote oxidation of these specific compounds evolved in pyrolysis, and possibly, formation of such species occurred in the oxidizer furnace.

Particulate Matter. Average size distributions of particulates, generated at 1100 °C and φ ) 1.46, are shown in Figure 9a. The largest yield of soot was found to correspond to fine particulates smaller than 0.4 µm, followed by particles in the range of 0.65–1.10 µm. Cumulative soot yields were found to be less than 20 mg/g of polymer injected, at all conditions tested, and they were nearly independent of the furnace temperature (see Figures 9b and 10a). The percentage of particulate matter smaller than 2 µm (PM2) over the total collected matter up to 10 µm monitored herein; i.e., PM2/ PM10, was found to be higher than 50%, as shown in Figure 10b. 4. Discussion In the following, the emissions from PS pyrolysis as well as the emissions from the ensuing indirect combustion, implemented in this work, are discussed and compared to emissions



Figure 8. Mole fractions (%) of CO2, CO, and O2 as monitored by online analyzers at the exit of the oxidizer furnace, along with superimposed calculated profiles, versus equivalence ratio.

Figure 9. (a) Particulate size distribution after pyrolysis at 1000 °C and combustion at 1100 °C. (b) Total soot yields (mg/g) after pyrolysis at 1000 °C and combustion at all three temperatures.

Figure 10. (a) Total soot yields (mg/g) from PS combustion at different equivalence ratios from either indirect (this work) or direct (previous work) combustion. (b) PM2/PMtotal (%) upon combustion at different equivalence ratios.

Figure 11. CO2 and CO yields (mg/g) from PS combustion at different temperatures and equivalence ratios from either indirect (this work) or direct (previous work) combustion.

from other pyrolysis or direct combustion data, either previously generated in this laboratory or reported in the literature. Moreover, the results obtained in this work on PS are compared with the emissions of PE pyrolysis and indirect combustion, recently obtained in this laboratory using the same equipment and analytical methods.22 4.1. Pyrolysis Yields. Kaminsky et al.11 pyrolyzed PS particles in a fluidized bed reactor at the temperature of 580 °C

and obtained 64.9% of styrene, 9.9% of other gases, 24.6% of oils, and 0.6% of residue. Scott et al.8 studied fast pyrolysis of PS in the temperature range of 532–708 °C and obtained ∼75% of styrene, 10% of other aromatics, and 15% of gases. In this present work the detected styrene yields were much lower, even lower than the methane or acetylene yields, and only represented few mg/g of polymer injected. Whereas the higher temperature implemented herein during polymer pyrolysis (1000 °C) could


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Figure 12. (a) Total light hydrocarbon (LH) yields, (b) acetylene yields, and (c) styrene yields, all in (mg/g), from PS combustion at different temperatures and equivalence ratios from either indirect (this work) or direct (previous work) combustion.

Figure 13. (a) Total PAH yields and (b) naphthalene yields (mg/g) from PS combustion at different temperatures and equivalence ratios from either indirect (this work) or direct (previous work) combustion.

have affected the composition of the pyrolysis products as compared to the literature,8,11 it is more likely this was caused by a deficiency in the sampling of styrene. Styrene was identified and quantified during the extraction of the XAD-4 resin and analysis of the semivolatile compounds. As styrene is one of the lighter aromatic compounds and it is, thus, very volatile, it could have been partially lost in the process of pouring the XAD-4 resin from the sampling stage into the glass bottles. Regarding the other semivolatile compounds, even though the composition of the pyrolysis effluent was different, the total PAH yields (29 mg/g) detected herein are of the same order of magnitude as in Panagiotou et al. (38 mg/g)14 and Panagiotou et al. (16.5 mg/g),13 who also continuously pyrolyzed polymer powders at 1100 and 1000 °C, respectively. 4.2. Pyrolysis Followed by Combustion Yields. The CO2 yields found in this work (indirect combustion) were consistent with those reported in prior work on direct combustion of PS, shown in Figure 11.21,23 Courtemanche et al.18 reported higher values but also implemented higher furnace temperatures (1220 °C). CO2 yields decreased as the equivalence ratio increased because of both increasing amounts of CO emissions released and increasing amounts of particulate unburned carbon at higher φs. At φ < 1 the CO yields were practically zero; however, in the fuel-rich region (φ > 1) CO increased drastically because of insufficient amounts of oxygen. In fact, the CO yields from the fuel-rich indirect combustion herein were higher than the yields previously detected in effluents of fuel-rich direct combustion of either PS or styrene. This may be explained on the basis of more uniform mixing of fuel and oxidizer in premixed fuel-rich flames, which facilitates the partial oxidation of intermediate species to CO instead of unburned hydrocarbons.

The LH emissions from fuel-lean indirect combustion of PS were comparable to those from direct combustion obtained elsewhere, as shown in Figure 12a. In the fuel-rich domain, the total LH yields from indirect combustion were generally higher than the yields from direct batch combustion reported by Pantalone et al.23 as well as by Ergut and Levendis,20 both also from steady-state steady-flow drop-tube furnace experiments. Yields of some compounds, such as acetylene and styrene, were comparable in the two combustion modes, as shown in Figure 12b,c. On the contrary, the indirect homogeneous combustion of PS generated drastically lower amounts of PIC as PAH and soot (see Figures 13 and 10, respectively). As presented in Figure 13a, the total PAH yields in the effluent of indirect combustion of PS were lower than the total PAH yields from direct combustion of PS; they were particularly lower than those from batch combustion of PS or styrene. As an example of a major PAH species, naphthalene followed the same trend as that of the total PAH (see Figure 13b). Figure 10b compares the proportion of PM2/total particulates found by Shemwell and Levendis4 from direct combustion of PS powders to that measured herein. In that work, conducted at 1027 and 1227 °C, only 20% and 40% of the particulate mass was smaller than 2 µm, respectively. In this work at least 50% of the particles were smaller than 2 µm, approaching 100% in near-stoichiometric conditions. The small size of the particles emanating from indirect combustion was attributed to the better contact of the pyrolyzates with air in the homogeneous nominally premixed flames encountered therein. Especially, in the fuel-lean domain, small nuclei of soot are created, but because they are well-dispersed, the growth to bigger particles is not favored. On the contrary, in direct combustion of PS particles or PS beds an oxygenstarved region is formed around the pyrolyzing polymer, at


the inner side of the diffusion flame. The large gradient of oxygen concentration between the bulk value and the negligible amount at the surface of the particle is responsible for the higher amount of products of incomplete combustion, such as PAH and soot, even at low overall (bulk) equivalence ratios. The total soot yields from indirect combustion and direct combustion of PS in different apparatuses are compared in Figure 10a. The particulate emissions from direct combustion of PS are at least 1 order of magnitude higher than those from the indirect combustion of the same polymer studied herein. Finally, it can be noted that in direct combustion of the polymer the same low yields as in the present work could be reached only when a ceramic barrier filter was used in the afterburner (see ref 2, stage 2). 4.3. Comparison of the Products of Indirect Combustion of Polystyrene (PS) and Polyethylene (PE). The same apparatus was used recently to study the pyrolysis and combustion of PE at the same temperatures (pyrolysis at 1000 °C and subsequent combustion at 900, 1000, and 1100 °C) and at nearly the same range of equivalence ratios.22 Moreover, a similar numerical simulation showed that the predicted lengths the particles travel to vaporize in the gasifier furnace are 9.76 and 9.6 cm for PE and PS, respectively, which are nearly identical. Pyrolysis of PE resulted in a large production of ethylene (accounting for 50% of the mole fraction of the total detected LH) and smaller amounts of others compounds (around 25% of methane and less than 10% of each of the others). Pyrolysis of PS resulted in a rather broad distribution of light hydrocarbons. Out of the measured LH yield, 45% was ethylene, 14% was benzene, 17% was acetylene, 15% was methane, and 5% was toluene, and the rest consisted of smaller amounts of other compounds. It has been already mentioned, however, that there were issues with the collection and quantification of styrene, which is expected to be the main product of PS pyrolysis. The major PAH from PE pyrolysis was naphthalene followed by acenaphthylene and phenanthrene. From PS pyrolysis the major PAH was phenanthrene, followed by biphenyl and naphthalene. The lack of CO yields in fuel-lean combustion of PS may be indicative that the mixing of the PS pyrolyzate gas with air in this apparatus was more effective than the mixing of the PE pyrolyzate gas with air. Underlining phenomena have been previously observed in this laboratory.3,14 PE particles have been observed to readily exhibit flash pyrolysis forming locally fuel-rich regions; that was not the case for PS particles. Furthermore, the higher CO2 yields upon combustion of PS, as compared to PE combustion yields, are in agreement with the aforementioned argument about mixing as well as with the difference in carbon content of the fuels, i.e., 92% for PS and 86% for PE. At fuel-lean equivalence ratios, the total LH yields of combustion were higher for PE than for PS. The PAH yields from combustion of PS are notably higher than those from PE, especially at fuel-rich conditions, which can be attributed to the aromatic nature of the former fuel. But in general, the PAH yields from either polymer were 1 order of magnitude lower than those from direct combustion of these fuels. PS produced 3 times higher soot yields than PE. Whereas PE particulates were found nearly quantitatively in the submicron size range ( 1 they increased drastically. The total light hydrocarbon and PAH yields in the products of combustion increased with equivalence ratio. Both the total PAH and the particulate yields from indirect homogeneous combustion of the PS pyrolyzates were an order of magnitude lower than the corresponding yields from direct combustion of this polymer. CO yields were higher, however, but this may be addressed in a practical system by introducing additional air. Phenanthrene and naphthalene were the most pronounced compounds in the combustion effluent, followed by acenapthylene. There were no clear trends of emissions with furnace temperature. The major PAH compounds formed in pyrolysis were drastically reduced after combustion; however, some minor compounds like biphenylene and dibenzofuran appeared to also have been generated in the second stage. Indirect homogeneous combustion of PS produced mostly submicron particulates, unlike direct combustion of the polymer


which, in addition to submicron particulates, also produced large amounts of highly agglomerated chains of supermicron particulates. Thus, indirect combustion produced larger relative yields of small particles than direct combustion did. However, the absolute yields of particulates from indirect combustion of PS pyrolyzates were drastically lower than the yields from direct combustion of the polymer throughout the monitored particle size distribution. The last two statements also apply to the combustion of PE, reported elsewhere. The largest amount of particulates produced from indirect combustion of PS was in the submicron range of 0–0.4 µm followed by particles from 0.65 to 1.10 µm.

GonçalVes et al.

Considering the results presented herein, gasification of PS followed by indirect homogeneous combustion of the pyrolyzates appears to be a valid technique for waste-plastics-to-energy generation with minimized emissions of pollutants. This was also the case in PE gasification and combustion examined earlier.22 Acknowledgment. We gratefully acknowledge Schlumberger Foundation for the grant “Faculty for the Future” 2005–2007. The authors acknowledge technical assistance from Jennifer Decoster, Ali Ergut, and Joseph Jordan. EF700431F

Emissions from Premixed Combustion of Polystyrene - Energy & Fuels

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