Ind. Eng. Chem. Res. 2004, 43, 2873-2886
2873
Laboratory Investigation of the Products of the Incomplete Combustion of Waste Plastics and Techniques for Their Minimization Zhenlei Wang,† Henning Richter,‡ Jack B. Howard,‡ Jude Jordan,§ Joel Carlson,§ and Yiannis A. Levendis*,† Department of Mechanical, Industrial, and Manufacturing Engineering, Northeastern University, 334 Snell Engineering Center, 360 Huntington Avenue, Boston, Massachusetts 02115, Department of Chemical Engineering, Massachusetts Institute of Technology, Building 66, 25 Ames Street, Cambridge, Massachusetts 02139, USA, and U.S. Army SBCCOM-Natick Soldier Center, Kansas Street, Building 3, Natick, Massachusetts 01760
This laboratory investigation aimed at reducing the emissions of pollutants from the batch combustion of three major waste plastics, polyethylene (PE), polystyrene (PS), and poly(vinyl chloride) (PVC). Results are reported herein on the emissions of CO, light aliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and particulates. Two-stage combustion of the polymers was investigated, with the first stage (primary furnace) operated at either 600 or 900 °C and the second stage (afterburner) at 1000 °C. Before exiting the afterburner, the effluent passed through a high-temperature ceramic filter, which retained and destroyed particulate products of incomplete combustion (PICs). The effect of the polymer quantity burned was examined, as well as the effect of mixing PS with PVC to explore the influence of chlorine on emissions of PICs. The results indicated that PVC produced the lowest amounts of all PICs, whereas PS produced the highest amounts of PAHs and particulates, and PE produced the highest amounts of CO, for the same mass of polymer burned. The combined effects of the afterburner treatment and the high-temperature filtration were consistently effective at minimizing all PICs when the primary furnace was operated at 600 °C. At that temperature, the emission yields at the exit of the filtered afterburner were lower than those at the exit of the primary furnace for the combustion of PS, PE, and PVC, respectively, as follows: CO by 70, 74, and 99%; particulates by 92, 99, and 99%; and PAHs such as fluorene by 96, 93, and 96%; phenanthrene by 73, 79, and 80%; fluoranthene by 86, 93, and 98%; and benzo[a]pyrene by 99, 96, and 99%. Model calculations, performed using a detailed kinetic network, indicated continued depletion of both PAHs and soot in the afterburner if their retention times therein were extended beyond those investigated in the absence of the ceramic filter. 1. Introduction Waste plastics are nonbiodegradable. The greatest quantities of plastics are found in containers and packaging (e.g., soft drink bottles, lids, shampoo bottles), but plastics also are found in durable goods (e.g., appliances, furniture) and nondurable goods (e.g., diapers, trash bags, cups and utensils, medical devices). The total mass of plastics in the U.S. municipal solid waste (MSW) stream in 2000 was 24.7 million tons and represented 10.7 wt % of the total MSW. Plastics constitute a rapidly growing segment of the MSW stream, increasing from under 1 wt % in 1960 to 10.7 wt % in 2000.1 During the past several years, the amount of plastics by mass in the MSW stream has stabilized at 11.5% in 1993, 9.5% in 1994, and 10.7% in 2000,1 as recycling streams have grown larger. By volume, plastics currently represent 24% of the MSW, due to their low density. Moreover, medical red-bag * To whom correspondence should be addressed. Address: 334 Snell Engineering Center, Northeastern University, Boston, MA 02115. Tel.: 1-617-373-3806. Fax: 1-617-373-2921. E-mail:
[email protected]. † Northeastern University. ‡ Massachusetts Institute of Technology. § U.S. Army SBCCOM - Natick Soldier Center.
(infectious) waste contains a much higher fraction of plastics, as high as 40 wt %. The current methods for dealing with the environmental problems resulting from this solid waste include source reduction, reuse, recycling, landfill, and wasteto-energy conversion. Most solid wastes are disposed through landfilling. However, with the lack of landfill space and with current challenges both in implementing the recycling of plastics and in finding markets for the recyclables, combustion of these materials in waste-toenergy (WTE) plants offers an alternative of technological and economic interest. Combustion of waste plastics provides a number of advantages, such as destruction of hazardous contaminants, reduction of mass and volume (by more than 90%), and energy recovery, as well as rendering the waste unrecognizable from its original form (which is often a requirement for medical waste).2 Moreover, the energy content (heating value) of most waste plastics, reported in Table 1, is comparable to that of premium fuels such as gasoline (44-47 MJ/kg). Various furnace designs are used for the combustion of MSW, such as rotary kilns, reciprocating grate stokers, fluidized beds, etc., and the resulting heat is used to generate steam and electricity. However, de-
10.1021/ie030477u CCC: $27.50 © 2004 American Chemical Society Published on Web 12/30/2003
2874 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 Table 1. Fuel Composition and Properties property
PS
PE
PVC
fixed carbon (%) volatiles (%) ash (%) carbon (%) hydrogen (%) sulfur (%) chlorine (%) nitrogen (%) oxygen (%) heat value (MJ kg-1)
1 99 0 92 8 0.04 0 0 0 44.5
0 100 0 86 14 0 0 0 0 40.5
9 91 1 38 5 0 57 0 0 19.2
pending on the operating conditions applied, not all of the fuel burns completely in a combustion system. Effective combustion requires fuel feed control for maintaining appropriate system operation, proper mixing of fuel and air, and maintenance of high temperature for a sufficient time (e.g., a recommended3 1000 °C for 2 s). The key challenge in waste combustion is to burn materials effectively while minimizing the emissions of hazardous products of incomplete combustion (PICs). Over the years, research has been conducted to study the pyrolysis and combustion behavior of plastics (see, for instance, Panagiotou et al.4-7), as well as to identify pyrolysates and combustion products (see, among others, refs 8-32). Results from the aforementioned works and from other studies have been summarized by Wang et al.33-35 Recent work in this laboratory has concentrated on the batch combustion of major waste plastics such as polyethylene (PE), polystyrene (PS), and poly(vinyl chloride) (PVC), as well as scrap automotive tires. In these studies, combustion conditions and techniques that minimize products of incomplete combustion (PICs) were assessed.33-38 The PICs were found to include CO, particulates (soot), and unburned hydrocarbons, such as polycyclic aromatic hydrocarbons (PAHs). Combustion of fixed beds of these solid fuels were conducted in two stages (furnaces), separated by an additional air mixing section in between. The primary furnace acted as a gasifier/burner, where polymer pellets were pyrolyzed and burned in diffusion flames. Upon mixing of the combustion effluent with additional air in a venturi mixer, the resulting gaseous charge was reacted homogeneously in the secondary furnace (afterburner). The effects of afterburning, of the operating temperatures, and of the gas compositions in both the primary and secondary furnaces on the release of PICs were investigated. It was found that the temperature of the primary furnace was very influential on the emissions of PICs, whereas the afterburner treatment was beneficial for many (but not all) PICs and at some (but not all) operating conditions. Oxygen enrichment in the afterburner and particulate filtration at the exit of the primary furnace were found to reduce PIC emissions. This work aimed at examining the effects of hightemperature filtration of the afterburner effluent, by placing a filter just before its exit, and at drawing conclusions on the overall operating conditions that minimize emissions of all PICs. The filter was expected to retain the particulates inside the high-temperature environment of the furnace for a prolonged period of time and, eventually, to destroy them by oxidation therein. The filter was also expected to partially retain condensed-phase PAHs or other lower-boiling-point PAHs that would adsorb to the particulate cake formed therein. The effect of increasing retention times of
organic species in the afterburner was explored by means of kinetic modeling. Furthermore, this study examined the effect of the amount of the polymer burned, and thus of the resulting global equivalence ratio, φ, on PIC emissions. Finally, it explored the effect of chlorine on the PIC emissions by mixing a chlorinecontaining plastic, i.e., PVC, with PS, as the pyrolysates of both of these fuels are mostly aromatic in nature. 2. Experimental Techniques and Procedure 2.1. Fuel Characteristics. In this study, three types of solid polymers were used. Granulated polystyrene (weight-average molecular weight ) 280 000 amu) and PVC (weight-average molecular weight ) 62 000 amu) were obtained from Aldrich. Low-density polyethylene pellets (LDPE, Dowlex) were obtained from Dow Chemicals, Midland, MI. The compositions of the fuels on a dry basis are reported in Table 1. 2.2. Experimental Apparatus for Staged Combustion. Combustion experiments were carried out in a 1-kW horizontal, split-cell, electric muffle furnace fitted with a quartz tube, 4 cm in i.d., 87 cm in length. The effluent of this primary furnace entered a secondary 2-cm-i.d., 38-cm-long muffle furnace (the afterburner) where further combustion occurred. A process flow diagram of the experimental system is shown in Figure 1. The experimental procedure has been described by Wang et al.33 Briefly, when the desired furnace temperature had been reached, a porcelain boat containing a small amount of sample (0.25-1.25 g) was inserted into the primary furnace through the tube’s entrance and placed in the middle of the quartz tube. The first half-length of this furnace acted as an air preheater. The air flow rate therein was 4 L min-1. As the sample devolatilized, under most conditions, combustion took place in a diffusion flame forming over the boat. Additional air (2 L min-1) was introduced between the two furnaces. To facilitate mixing of the combustion effluent with the additional air, a venturi mixer was installed in the quartz tube prior to the exit of the primary furnace. At the periphery of the venturi, four radially positioned perpendicular jets of additional air were discharged (see Figure 1), preheated to approximately 100 °C below the furnace wall temperature. Calculations on the momentum flux ratios of jets in the crossflow, accounting for measured temperatures at the venturi centerline and at the jet exits, showed that the four jets placed perpendicularly to the venturi effectively penetrated the effluent stream to the centerline of the venturi.33 This suggested good mixing of the effluent gas stream from the primary furnace with the additional air. The primary furnace wall temperature was set to 600 or 900 °C, and the secondary wall furnace temperature was kept constant at 1000 °C. Gas temperatures in the absence of a flame were measured by means of a thermocouple and were found to be fairly constant for most of the length of these furnaces, ∼25 °C below the wall temperatures. The temperature in the tube section between the two furnaces dipped to 250-300 °C. After leaving the primary furnace, one-half of the effluent passed through the first sampling stage, while the other half was introduced to the secondary furnace. Prior to the exit of the secondary furnace, a ceramic barrier filter was inserted in the quartz tube (see Figure 1). This was a honeycomb wall-flow filter made of sintered silicon carbide (SiC; see Figure 2). This filter was not catalyzed. Ceramic monoliths were shown, by previous work in this
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 2875
Figure 1. Schematic of the experimental two-stage laminar-flow reactor, fitted with a ceramic (SiC) honeycomb filter monolith prior to the exit of the second stage (afterburner).
Figure 2. (Left) Frontal view of a large silicon carbide (SiC) wall-flow monolith filter, manufactured by Ibiden. (Right) Scanning electron micrographs of the microstructure of the ceramic walls in the channels of this honeycomb monolith. A uniform pore size distribution is evident and was confirmed by measurements.
laboratory,39 to be very effective in retaining combustion-generated particulates. Larsen et al.40 tested SiC filters and found a particulate capture efficiency of 97% for submicron particles (soot agglomerates). Diesel soot spherules are very fine, on the order of 30 nm;41 agglomerated soot aerodynamic diameters are typically under 1 µm.42 Ceramic filters can withstand high temperatures without experiencing failures; SiC can safely reach temperatures as high as 2000 K. Nevertheless, such high temperatures are not needed in this application as soot ignites at temperatures as low as 600 °C42 in the presence of oxygen. According to the manufacturer’s (Ibiden) data, SiC filters have a high chemical resistance and should not be affected by possible soot constituents or minerals (for example, zinc oxides or sulfates) that can vaporize in the volatile flame and pass through the filter. In a practical application, any accumulated ash can be periodically removed from the filters by back-pulsing.39,40 2.3. Combustion Emissions Monitoring. Polycyclic aromatic hydrocarbon (PAH) emissions, as well as emissions of CO, CO2, and particulates (soot and tars, but also unburned condensed oligomers) from the combustion of polyethylene, polystyrene, and PVC were measured at the exits of the two furnaces by trapping both the condensed and the gas-phase compounds. Sampling stages were placed at the exits of the two furnaces (see Figure 1), each handling one-half of the flow of the combustion products. Before being sampled, the effluent was diluted with 2 L min-1 nitrogen gas flowing between two concentric tubes, the innermost of which was perforated. Graseby sampling heads were
used with a filter stage and a glass cartridge containing Supelco XAD-4 adsorbent. The particulate emissions were trapped on 90-mm-diameter, 1-mm-thick Fisherbrand paper filters with a nominal pore size of 0.45 µm. The particulate yields were derived gravimetrically by weighing each filter before and after every experiment. No significant moisture uptake was observed during the weighing process. Gas-phase PAHs were adsorbed onto the bed of XAD-4 resin. The length of the XAD-4 bed was more than twice its diameter for effective trapping of PAHs. At the exits of the sampling stages, a dual infusion/withdrawal syringe pump was set up to extract gas emission samples simultaneously into two glass syringes (1 mL). To obtain the average amount emitted during the transient combustion process, sampling was performed throughout the duration of the combustion by preadjusting the withdrawal flow rates. Sampling commenced at the onset of the luminous flame or, at the absence of ignition, when a white aerosol was detected at the exit of the furnace. Sampling was terminated as soon as this flame extinguished or when the pyrolysis-generated aerosol ceased to be produced. The gaseous effluent samples extracted with the syringes were analyzed by gas chromatography using a Hewlett-Packard instrument (HP6890), equipped with a flame ionization detector (FID). The capillary column was HP-5 type, cross-linked 5% PH ME siloxane, with a length of 30 m, an inside diameter of 0.320 mm, and a film thickness of 0.25 µm. Samples were introduced through the split/splitless inlet. By matching the retention times of the samples with the retention times of known compounds, several aliphatic compounds could
2876 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004
be identified. The gas chromatographic conditions were as follows: oven temperature of 80 °C, 25-min holding time followed by 2-min holding time for postrun. The back inlet temperature was 280 °C, and the detector temperature was 300 °C. The instrument was calibrated regularly with two Scotty IV analyzed gas mixtures containing C1-C4 hydrocarbons once per week. After the moisture had been removed with a mildly heated Permapure dryer, CO/CO2 were monitored using Horiba infrared analyzers, and O2 was monitored using a Beckman 350 paramagnetic analyzer. The output was recorded using a Data Translation DT-322 board in a microcomputer. The integrated acquisition system used DT VPI within a Hewlett-Packard HP VEE visual programming environment. The signals from the gas analyzers were recorded during the duration of the experiment; subsequently, these data were converted to partial pressures and, upon numerical integration, to mass yields. 2.4. Extraction and Concentration of PAH Emissions. After the combustion experiments had been completed, the filters and XAD-4 resins were removed, placed in separate glass bottles with Teflon-lined caps, and stored in a freezer at 4 °C until analysis. Before extraction, a 50-µL internal standard mixture containing 100 µg each of naphthalene-d8, acenaphthene-d10, anthracene-d10, chrysene-d12, and perylene-d12 was directly added to the filters, as well as to the resins in each bottle. Blank combustion experiments were also performed, during which the furnace was operated in the presence of the XAD-4 resins and filter, but in the absence of fuel. Target compounds that appeared in any of the blanks were appropriately qualified. A Dionex ASE 200 accelerated solvent extractor was used for extracting the organic compounds from both the XAD-4 resins and the cellulose filter papers. The XAD-4 resins were transferred to 33-mL extraction cells, whereas the filter papers were transferred to 11-mL extraction cells. The extraction cells were allowed to equilibrate initially at 40 °C in the ASE 200 system for 1 min. The extraction cells were then filled with methylene chloride and allowed to thermally equilibrate at 40 °C; the cell was pressurized to 500 psi for a period of 15 min. Following the 15-min soak time, the cells were each flushed with a quantity of fresh methylene chloride equivalent to 80% of the cell volume and finally purged for 90 s with nitrogen. The methylene chloride extracts were collected in separate bottles for concentration. Two extraction cycles were used per cell. The total extraction time for the two-cycle process was around 25 min. About 45 mL of methylene chloride was used for the XAD-4 resins, whereas 20 mL was used for the extraction of the filter papers. No more than 30 mL of XAD-4 resin could be placed within a 33-mL extraction cell because of the expansion of this resin in methylene chloride. The samples were concentrated under vacuum to a final volume of 10 mL for analysis by gas chromatography coupled with mass spectrometry (GC-MS). 2.5. PAH Analysis by Gas Chromatography Coupled with Mass Spectrometry (GC-MS). The analytical studies were conducted in accordance with EPA Method 8270A, as specified in the analytical methodology SW846. The method was simplified to remove surrogate analysis procedures in the absence of potential matrix effects normally observed in environmentally obtained soil and water samples. Analytical data were reviewed according to U.S. Environmental
Protection Agency (EPA) Volatile/Semivolatile Data Validation Functional Guidelines, http://www.epa.gov/ superfund/programs/clp/guidance.htm. Any analytical data that failed to meet those standards were rejected, and the combustion experiments were repeated. The GC-MS system consisted of a Hewlett-Packard (HP) model 6890 GC equipped with an HP model 5973 massselective detector. The GC-MS conditions and data reduction procedure were described previously.26,27,33 The instrument was tuned in accordance with EPA semivolatile criteria prior to the GC-MS analysis of each set of samples. The instrument passed initial and continuing calibration criteria. Each of the target compounds, as well as the tentatively identified compounds, was quantified using the appropriate deuterated internal standard. In a departure from the EPA method, the GC-MS system was run in the full-scan mode and not in a single-ion-monitoring mode. This was done to ensure the identification, quantification, and reporting of tentatively identified compounds. The use of the full-scan mode does not significantly modify the method except to raise the lower reporting limit to about 1 µg of component per gram of fuel burned. Values that are less than 1 µg/g are technically not considered to have been detected. Analysis of the combustion system blanks indicated the presence of xylenes, bis(2-ethylhexyl) phthalate, and siloxanes in quantities sufficient to reject all positive results for these compounds in the combustion extracts. The standard solutions containing 100 µg each of five characteristic standards were diluted to 10 mL and were analyzed to provide both an instrument blank and an indication of the extraction efficiency of each of the internal standards. Samples with an extraction efficiency of less than 50% for any of the internal standards were repeated. The extraction recovery was in the range of 85-100%, which is in agreement with the U.S. EPA national functional guidelines http:// www.epa.gov/superfund/programs/clp/guidance.htm, recommending a recovery of greater than 50% for the internal standards. The average recovery and statistical evaluation of the internal standards have been discussed elsewhere (ref 37, Table 1). Remarkably the percent difference (ratio of standard deviation to average recovery) was very low, demonstrating the excellent reproducibility of this extraction technique for the analysis of XAD-4 resins and filter papers. To assess the reproducibility of the PAH concentrations, experiments were conducted in triplicate. Average values are presented in this paper. The typical agreement in this type of experimental data was discussed in refs 33 and 34. Uncertainties arise from a combination of combustion variability and the sampling, extraction, concentration, and analytical techniques. The experimental procedure, however, was kept consistent in all evaluations to ensure the validity of relative trends. 3. Results and Discussion Batches of 0.25 and 0.5 g of PE, PS, and PVC were burned in these experiments. Also, batches of 1.25 g of PVC were burned to provide tests in which the amount of combustible mass in PVC (i.e., excluding chlorine) was approximately equal to that in the 0.5 g PS samples. Moreover, two mixtures of PVC with PS were burned, one consisting of 0.375 g of PS and 0.125 g of PVC (i.e.,
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 2877 Table 2. Transient Combustion Durations and Average Global Equivalence Ratios, O PS mass 0.25 g 0.5 g 1.25 g 0.375 g PS + 0.125 g PVC 0.375 g PS + 0.3 g PVC
PE
PVC
temp (°C)
combustion time (s)
φ
combustion time (s)
φ
900 600 900 600 900 600 900 600 900 600
61 74 63 87
0.68 0.56 1.31 0.95
60 111 67 127
0.77 0.41 1.37 0.72
75-25% by total mass) and another consisting of 0.375 g of PS and 0.3 g of PVC (i.e., 75-25% by mass of combustibles, i.e., excluding chlorine). After a fixed bed of each sample had been placed at the midpoint of the length of the preheated primary furnace, the fuel bed was heated and devolatilized. Under most conditions, the pyrolysates ignited, and a diffusion flame formed over the fuel bed. Time-dependent evolution profiles of the major products (CO and CO2) exhibited double peaks at 600 °C, indicative of the sequential pyrolysis of species, particularly in the cases of PS and PVC (see also refs 4-6). The smaller sample of PVC (0.25 g) exhibited ignition difficulties at 600 °C. The durations of combustion, as determined from the transient CO2 profiles, are listed in Table 2. They ranged from 1 to 2 min and became longer as the sample mass increased. Moreover, the combustion durations were longer at 600 °C than at 900 °C for PE and PS, but not always for PVC. In fact, during single-particle combustion experiments, Panagiotou and Levendis6 observed that higher furnace temperatures resulted in shorter combustion times for PE (as expected), but surprisingly longer combustion times for PVC. The irregular behavior of PVC was thought to be due to the high concentration of chlorine in the volatiles that escape during the early stages of pyrolysis. From the fuel mass burned and the combustion durations, global (overall) equivalence ratios, φ, averaged over the transient combustion period, were calculated and are included in Table 2. The average global φ values in most cases reflected fuel-lean conditions (φ < 1), with the exception of the combustion of 0.5 g of either PE or PS at 900 °C, where overall fuel-rich conditions prevailed (φ > 1). 3.1. Oxygen, Carbon Dioxide, and Carbon Monoxide Profiles. In all cases, the minimum oxygen concentrations at the exit of the primary furnace were fairly high (7-20%). These values were much higher than expected on the basis of the average global equivalence ratios (listed in Table 2), especially for the aforementioned fuel-rich cases. This is because additional air was introduced at the mixing venturi, which is located prior to the exit of the primary furnace (Figure 1). The minimum oxygen partial pressures at the exit of the secondary furnace were lower than those at the exit of the primary furnace for all three polymers, which indicates that additional oxidation of fuel pyrolysates occurred in the secondary furnace (see Figure 3a). The gap between the oxygen partial pressures of the primary and secondary furnaces became wider as the sample mass increased, indicating that the primary furnace treatment released increasing amounts of products of incomplete combustion to the afterburner. The mini-
mixtures
combustion time (s)
φ
67 55 65 56 82 110
0.26 0.31 0.53 0.61 1.05 0.78
combustion time (s)
φ
90 82 86 118
0.72 0.79 1.01 0.74
mum oxygen partial pressures during combustion of PVC were much higher than those recorded during the combustion of the other two polymers, reflecting the lower combustible (C and H) mass per unit sample weight of PVC as more than half of PVC is chlorine. When the combustible mass of PVC (1.25 g) was approximately equal to the mass of PS (0.5 g), the exit oxygen partial pressures were comparable. The yields of CO2 were obtained by integrating the time-dependent profiles of the on-line analyzers; they are shown in Figure 3b. As expected from the above discussion on oxygen, in all cases, the yields of CO2 from the afterburner were higher than those from the primary furnace. Moreover, in the cases of PE, PS, and the mixtures, the yields of CO2 at the exits of both furnaces were higher when the primary furnace was operated at 600 °C than when it was operated at 900 °C. This is indicative of better combustion at the lower temperature. PVC produced less CO2 than PS and PE at 600 °C. Moreover, the CO2 yields from PVC at the exit of the primary furnace were lower at 600 °C than at 900 °C, as ignition difficulties were experienced at the low temperature. However, additional oxidation in the afterburner equalized the CO2 yields at its exit for both primary furnace temperatures. The yields of CO for combustion of PS, PE, and PVC are presented in Figure 3c. PE produced the highest CO emissions, as measured at the exits of both furnaces, followed by PS and then PVC. The high CO yields from PE can be attributed to its mode of combustion, which often generated “puffs”, as documented previously;7 the low yields from PVC can again be associated with its low combustible mass and the low global equivalence ratios (Table 2). The CO yields from the combustion of the PS/PVC mixtures were slightly below those of PS and considerably higher than those of PVC for comparable sample masses. When the primary furnace was operated at 600 °C and the pyrolysate fluxes from PS and PE were low, resulting in fuel-lean global equivalence ratios, the afterburner reduced the concentrations of the CO emitted from the primary furnace. To the contrary, when the primary furnace temperature was 900 °C, additional CO was generated in the afterburner, as fuel-rich conditions prevailed therein during the middle portion of the transient combustion period. 3.2. Emissions of Light Aliphatic Hydrocarbons. Light aliphatic hydrocarbon emissions included methane, acetylene, ethylene, ethane, propylene, and propane; the yields of these species are shown in Figure 4. PE produced the highest yields of all of these species, especially ethylene; PVC was second in nearly all cases.
2878 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004
Figure 3. (a) Minimum O2 partial pressures at the exits of the primary and secondary furnaces, (b) CO2 yields (mg/g), (c) CO yields (mg/g), (d) cumulative PAH yields (mg/g), (e) cumulative particulate yields (mg/g), at primary furnace temperatures of 600 and 900 °C. In all cases, Tafterburner ) 1000 °C. Yields for 0.25 and 0.5 g of PS, PE, and PVC are shown, as well as yields for 1.25 g of PVC and two mixtures of PS and PVC.
With the exception of methane in some cases, the mole fractions of these species were reduced in the afterburner. 3.3. Yields of PAHs from the Combustion of Polystyrene, Polyethylene, and PVC. Whereas most polycyclic aromatic hydrocarbons (PAHs) do not show mutational activity, some are known carcinogens (benzo[a]pyrene), and a larger number have been shown to cause mutations of single cells, both bacterial and human.43 The cumulative PAHs are shown in Figure 3d, and individual species for all cases are shown in Figure 5. Under most conditions, combustion of PS produced the highest amount of PAHs, as monitored at the exit of the primary furnace, followed by PE and then PVC. Mixtures of PS and PVC released amounts of PAH emissions between those released from the two polymers burned alone. In most cases, PAH emissions increased with increasing temperature of the primary furnace. Naphthalene was the most pronounced PAH in the combustion effluent of PE and PVC. However, in the case of PS, phenanthrene was also produced in large quantities, which often exceeded those of naphthalene. This finding is consistent with the possible role of phenylacetylene as a phenanthrene precursor,44 formed from styrene monomer in the present case. Burning larger sample amounts of PVC increased the emissions
of most PAH species, with the notable exception of naphthalene. The afterburner/filter treatment was effective at reducing all PAH emissions when the primary furnace was operated at 600 °C, whereas at 900 °C, mixed results were obtained. Nevertheless, the reduction of PAH concentrations by the filtered afterburner for the 600 °C primary furnace temperature was consistently remarkable for all polymers and for all PAH species. Dramatic reductions in individual PAH species concentrations were achieved, as evidenced in Figure 5 and Table 3, with the exception of naphthalene, the reduction of which was less pronounced. 3.4. Yields of Particulates from the Combustion of Polystyrene, Polyethylene, and PVC. The particulate yields at the exits of the two furnaces are shown in Figure 3e. The particulate matter was soot, i.e., carbonaceous material and extractable polycyclic aromatic hydrocarbons (PAHs). The trends in the concentration of soot parallel those of most large PAHs, in agreement with findings reported elsewhere.45 Indeed, PS produced the highest yields of particulates from the primary furnace, much more than PE and PVC. The fact that the combustion of PS produces copious amounts of soot was observed and quantified previously, and soot was found to be highly agglomerated.5,29 In previous
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 2879
Figure 4. Yields of the light hydrocarbons methane, acetylene, ethylene, ethane, propylene and propane (all in µg/g) at the exits of the primary and secondary furnaces as a function of the primary furnace temperature. In all cases, Tafterburner ) 1000 °C. Yields for 0.25 and 0.5 g of PS, PE, and PVC are shown, as well as yields for 1.25 g of PVC and two mixtures of PS and PVC.
work, one-third of the mass of soot agglomerates from the combustion of PS was found to be PM2, i.e., 2 µm or smaller.29 The size of the individual spherules comprising the soot agglomerates ranged from 0.05 to 0.2 µm.
In contrast, PE produced fine soot, as more than threequarters of the mass of soot agglomerates from the combustion of PE was found to be PM2. Spherule sizes ranged between 0.1 and 0.2 µm. PVC produced the
2880 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 2881
Figure 5. 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, Tafterburner ) 1000 °C. Yields for 0.25 and 0.5 g of PS, PE, and PVC are shown, as well as yields for 1.25 g of PVC and two mixtures of PS and PVC.
lowest yields of soot in a rather wide spectrum of agglomerate aerodynamic sizes (less than one-third
were PM2).29 Similarly to CO and PAH yields, the yields of particulates typically increased with increasing pri-
2882 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 Table 3. Reductions in the Emissions of PICs material
PS
temperature(°C) emissions CO CO2 particulates total PAHs naphthalene biphenyl acenaphthalene fluorene phenanthrene anthracene fluoranthene cyclopenta[c,d]pyrene benzo[a]pyrene
PE
600
900
PVC
600
900
600
900
2′/1a
2′/2b
2′/1a
2′/2b
2′/1a
2′/2b
2′/1a
2′/2b
2′/1a
2′/2b
2′/1a
2′/2b
0.84 1.77 0.01 0.40 0.91 0.04 0.06 0.00 0.02 0.00 0.02 0.00 0.01
0.31 1.09 0.08 0.76 0.98 0.27 0.09 0.04 0.27 0.07 0.14 0.05 1.00
5.12 2.18 0.20 0.43 0.70 0.10 0.98 0.05 0.28 0.23 1.63 5.06 1.52
1.19 1.55 0.26 0.47 0.50 0.31 0.21 0.26 0.79 0.56 0.61 0.94 0.44
0.66 1.42 0.00 0.46 0.72 0.08 0.04 0.02 0.05 0.01 0.02 0.01 0.04
0.26 1.22 0.00 0.54 0.58 0.25 0.21 0.07 0.21 0.07 0.07 0.03 0.19
2.27 1.74 1.58 1.57 1.07 0.94 2.15 1.61 2.69 1.66 4.80 12.48 5.93
2.26 1.58 0.44 0.43 0.50 0.32 0.28 0.26 0.50 0.36 0.57 0.29 0.44
0.00 4.57 0.00 0.63 0.91 0.04 0.19 0.00 0.03 0.00 0.02 0.00 0.00
0.00 1.90 0.00 0.65 0.68 0.34 0.12 0.04 0.20 0.04 0.02 0.00 0.00
0.10 1.52 0.00 0.83 1.01 0.16 0.22 0.05 0.07 0.00 0.03 0.00 0.00
0.44 1.89 0.00 1.08 0.98 0.67 0.76 0.05 0.50 0.00 0.41 0.00 0.00
a Ratio of emissions at the exit of the afterburner fitted with the filter to those at the exit of the primary furnace (2′/1). b Ratio of emissions at the exit to afterburner fitted with the filter to those at the exit of the afterburner without the filter (2′/2).
mary furnace temperature. The soot emissions from the mixtures were lower than those from PS. The combination of the afterburner with the hightemperature barrier filter dramatically reduced the particulate emissions in all cases. Especially when the primary furnace was operated at 600 °C, the particulate emissions were nearly eliminated by the afterburner treatment. This indicates that 600 °C would be an optimum primary furnace temperature for minimizing particulate emissions for PS, PE, and PVC. One should keep in mind that soot formation is more limited at this lower primary furnace temperature. A decrease in the burning rate of the fuel can reduce the effective fuel/ air ratio in the flame, thus reducing the soot yield.46,47 Lowering the flame temperature reduces the gas-phase pyrolysis rate, the formation of precursors, and thus the incipient soot formation rate. This reduces the soot yield.48 Physical and chemical processes for soot formation and oxidation are very complex. There is no single characteristic parameter that determines the amount formed per unit weight of fuel consumed27 because the amount of soot formed depends both on the particular fuel and on the overall combustion process. The most dominant physical characteristic that affects soot formation is the temperature-time history of the pyrolyzing fuel.48-51 Glassman49 concluded that, in diffusion flames, the higher the flame temperature, the greater the tendency to form soot. Moreover, the fuel structure plays an essential role in the sooting tendency of diffusion flames. The formation of the initial rings controls the rate of incipient soot formation, and the particle concentration determines the amount of soot formed. More specifically, the rate of formation of the first aromatic ring is the rate-controlling step in soot formation. 4. Overall Discussion and Significance of the Results This work presents a technique for minimizing the products of incomplete combustion (PICs) from PS, PE, and PVC polymers (waste plastics). It consists of twostage combustion process in conjunction with an intermediate venturi mixer for the introduction of additional air. The exit of the afterburner is filtered by a hightemperature ceramic barrier filter. In this configuration, the filter is self-regenerating, as carbonaceous particu-
lates collected therein burn at the filter’s soaking temperature. Had ash accumulated in the filter, then aerodynamic regeneration, as described in refs 39 and 40, would have been necessary. This treatment was found to be consistently effective in reducing emissions of PICs when the primary furnace was operated at a relatively low temperature, in the vicinity of 600 °C. This result is in agreement with the findings of previous studies in this laboratory, involving the burning of a variety of solid waste fuels.33-38 Table 3 summarizes the reductions in emission yields experienced by this staged combustion technique. For all three polymers (PS, PE, and PVC), Table 3 presents the ratios of (a) the PIC yields at the exit of the afterburner fitted with the filter (2′) to those at the exit of the primary furnace (1) and (b) the PIC yields at the exit of the afterburner fitted with the filter (2′) to those at the exit of the afterburner without the filter (2), which were obtained in previous work.35 The primary furnace was operated at either 600 or 900 °C, and the afterburner was operated at 1000 °C, In case a, i.e., ratios of 2′ to 1, the filtered afterburner reduced the emissions from the three polymers burning in the primary furnace operated at 600 °C as follows: emissions of CO by 14-99%, emissions of particulates by 99-100%, emissions of fluorene by 98-99%, emissions of phenanthrene by 95-98%, emissions of fluoranthene by 98%, emissions of cyclopenta[c,d]pyrene by 99%, and emissions of benzo[a]pyrene by 94-99%. Total PAHs were reduced to a lesser extent, 37-60%, only because the reduction of naphthalene emissions was not as pronounced as the reductions of the rest of the PAH species. In case b, i.e, ratios of 2′ to 2, the PIC emissions of the filtered afterburner were lower than those of the unfiltered afterburner (again for the case of the 600 °C primary furnace temperature), for the three polymers as follows: emissions of CO by 74-99%, emissions of particulates by 92-99%, emissions of fluorene by 9697%, emissions of phenanthrene by 73-80%, emissions of fluoranthene by 86-98%, emissions of cyclopenta[c,d]pyrene by 95-99%, and emissions of benzo[a]pyrene by 0-99%. Total PAHs were reduced by a lesser amount, 24-46%, again because the reduction of naphthalene emissions was not as dramatic as the reductions of the rest of the PAH species. Thus, whereas the afterburner itself was found to beneficial in reducing the yields of
Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 2883 Table 4. Emission Ratios from Burning Two Different Sample Masses (0.25 and 0.5 g) as Monitored at the Exit of the Afterburner material temperature (°C) CO CO2 particulates total PAHs naphthalene biphenyl acenaphthalene fluorene phenanthrene anthracene fluoranthene cyclopenta[c,d]pyrene benzo[a]pyrene
PS
PE
PVC
600
900
600
900
600
900
(0.00/39.85)a 0.91 (0.00/1.00)a 0.82 0.73 1.21 0.27 25.79 4.84 29.08 2.37 3.60 0.37
0.98 1.22 0.79 0.91 1.69 0.36 0.57 0.44 0.33 0.50 0.48 1.03 0.57
(0.00/69.47)a 1.07 (10.13/0.00)a 2.30 2.32 1.59 0.11 5.65 1.34 3.66 2.55 1.36 0.10
0.95 1.31 0.51 0.82 1.22 0.36 0.34 0.38 0.77 0.89 0.50 0.66 0.39
(0.00/0.00)a 1.11 (0.00/0.00)a 1.41 1.40 1.13 0.18 10.68 1.07 2.42 10.85 (0.08/0.00)a (0.07/0.00)a
(0.00/5.68)a 0.98 (0.00/0.00)a 2.09 2.36 0.81 0.07 10.85 1.40 (0.31/0.00)a 3.46 (0.08/0.00)a (0.15/0.00)a
a If the mass of one or both of the displayed species is below the detection limit, then emissions from both polymer sample quantities (0.25 and 0.5 g) are presented.
PICs from the primary furnace, it was the hightemperature ceramic filter (placed inside the afterburner) that produced remarkable additional reductions. This dramatic reduction in PAHs and soot by the high-temperature barrier filter was attributed to the retention of these species therein for a prolonged period of time (longer than the 0.7-s calculated residence time in the afterburner without the filter). Condensed species (soot and heavy PAHs) were trapped in the filter, and additional PAHs are expected to have been adsorbed (physisorbed or chemisorbed) on the particulate cake therein. The amount of additional retention time of these species in the afterburner furnace afforded by the filter cannot be easily quantified. Species deposit throughout the combustion time, and thus experience variable retention times inside the hot filter. Thus, to explore the fate of PAHs in the afterburner/filter environment at the prolonged retention times therein, numerical modeling was conducted (see the following section). Decreasing the amount of fuel burned during the batch combustion did not reduce the combustion times proportionately. The global equivalence ratios were reduced, but the yields of PICs did not always decrease, possibly because of still high local equivalence ratios. The emission ratios from burning two different sample masses, 0.25 and 0.5 g, as monitored at the exit of the afterburner, are shown in Table 4. A high ratio would signify higher emissions from the 0.25 g sample, whereas a low ratio would signify higher emissions from the 0.5 g sample. One can seen that the CO yields were dramatically reduced when the combusted sample mass was halved. The particulate yields were also reduced in most, but not all, cases. The yields of individual PAH species increased in most cases, but decreased in some. Generally, the reduction of PICs upon halving of the sample mass in batch combustion was not found to be universal under the conditions examined. PVC produced the smallest yields of PICs on the basis of both total mass and mass of combustible content (i.e., excluding chlorine). This finding might be contrary to the notion that the presence of chlorine in flames enhances the formation of soot by hydrogen abstraction.50,51 However, one should keep in mind that the mode of combustion herein involves transient diffusion flames. As chlorine has been identified to be the first of the pyrolysates to evolve, it might have partially diffused and transported away to the bulk gas before ignition and establishment of a flame takes place. If this hypothesis is correct, as ignition is delayed during the
dechlorination process by the scavenging of radicals to form HCl, oxygen diffuses inward and mixes with the organic devolatilizates. The ensuing flame has a degree of premixing. This argument is supported by the experimental findings of ref 5 and 6. Fuel and air premixing has beneficial effects in preventing the formation of PICs. Burning mixed-polymer wastes of PS and PVC was beneficial in reducing the PIC yields relative to those from pure PS. Consistent with the above arguments, when the amount of PVC was increased while the amount of PS was kept constant, i.e., for (0.3 g PVC + 0.375 g PS) vs (0.125 g PVC + 0.375 g PS), most of the PIC yields were somewhat reduced. 5. Assessment of Longer Afterburner Treatment by Means of Kinetic Modeling A significant reduction of the release of pollutants, such as soot and PAHs, from the combustion of waste plastics by means of the optimization of furnace operating conditions has been shown in previous laboratory studies.33-35,38 In the case of the combustion of such solid materials, the formation of at least some PAHs and soot cannot be avoided, due to the occurrence of diffusionflame-type combustion. However, afterburner treatment, upon injection of additional air, was found to allow for at least partial depletion of most of them. Afterburner treatment achieving a significant reduction of initially formed PAHs and particulates, followed by filtration removing the remaining material, can be considered as a suitable approach for environmentally friendly incineration of waste plastics. However, efficient operation of the afterburner, requires a good understanding of the impact of all critical parameters. For instance, the effect of the mixture composition at the entrance of the afterburner has been investigated for variations in the type of fuel studied,33-35,37,38 the temperature of the primary furnace,33 and the addition of pure nitrogen or oxygen (instead of air) at the venturi.36 The temperature of the afterburner was also found to be an important parameter.34,36 Significant amounts of oxygen were found to remain in the afterburner at any time of the experiment, indicating that the increase of reaction time should lead to a continuation of the PAH and soot oxidation process. Taking into account the complete or nearly complete absence of monomers (of these polymers), oligomers, and other potential PAH and soot precursors at the exit of
2884 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004
Figure 6. Model predictions for acenaphthalene, phenanthrene, pyrene, and benz[a]pyrene at the exit of the afterburner. Combustion of PE was assumed to take place in the primary furnace at Tprimary furnace ) 600 °C. Tafterburner) 1000 °C. (a) Acenaphthalene, (b) phenanthrene, (c) pyrene, (d) benzo[a]pyrene.
the afterburner, the competition between the formation of PAHs and soot and their oxidation is expected to be increasingly dominated by the latter process with increasing residence time in the afterburner. To investigate the effect of residence time in the afterburner on the PAH and soot concentrations, kinetic modeling was used because it allows for the expedient assessment of a large number of conditions and, therefore, is suitable for the identification of operating conditions of potential interest. A detailed kinetic model taking into account the pressure dependence of chemically activated reactions and using the best available thermodynamic and kinetic property data has been developed in recent years. It has been tested exhaustively for different fuels in premixed flames54-56 and well-stirred/plug-flow reactor setups.57 The updated atmospheric-pressure version of the model57 has been used previously for the description of PAH formation and depletion in the afterburner of two-stage incineration of polystyrene (PS)33-35 and monomeric styrene.37 The afterburner was approximated as a plugflow reactor using the Chemkin software package.58 For most species, i.e., CO, CO2, PAHs,33,34,37 and major light hydrocarbons,35 experimental data were used for the composition of the inlet mixture, whereas for some species, such as H2O, for which such data were not experimentally available, the concentrations at the inlet of the afterburner were estimated by means of a detailed modeling approach.33-35,37 Recently, details on the conversion of PAHs to soot, as well as PAH oxidation, obtained using experimentally determined global kinetic data,36 have been added to the model.37 In the present work, the modeling of recent experiments involving the combustion of waste polyethylene (PE) at a primary furnace temperature of 600 °C and an afterburner temperature of 1000 °C is extended. The effect of the extension of the residence time to 20 s was investigated, and a continued decrease of all PAHs was observed (Figure 6). Confirming the predictive capability of the kinetic model, the experimental concentrations of the investigated species, observed at a residence time of ∼0.7 s,35 are in reasonable agreement with the
corresponding model predictions. In addition, soot oxidation was included in the model using a rate constant estimated on the basis of the work of Nagle and Strickland-Constable.59 Soot formation was found to continue up to a residence time of ∼1 s, and complete soot depletion was achieved at high residence times. This finding is of particular interest taking into account the fact that a residence time of only ∼0.7 s has been investigated experimentally when the afterburner is operated without the filter. In conclusion, continued depletion of PAHs and soot can be expected with increasing residence times in the furnace under the conditions implemented in this work. Literature Cited (1) See: http://www.epa.gov/epaoswer/non-hw/muncpl/plastic. htm. (2) Seeker, R. Combustion By-Product Formation: An Overview. In Proceedings of the Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 867-885. (3) Lemieux, P. M.; Ryan, J. V.; Bass, C.; Barat, R. Emissions of Trace Products of Incomplete Combustion from a Pilot-Scale Incinerator Secondary Combustion Chamber. J. Air Waste Manage. Assoc. 1996, 46, 309-316. (4) Panagiotou, T.; Levendis, Y. A. A Study on the Combustion Characteristics of PVC, Polystyrene, Polyethylene, and Polypropylene Particles under High Temperature. Combust. Flame 1994, 99, 53-74. (5) Panagiotou, T.; Levendis, Y. A.; Delichatsios, M. A. Combustion Behavior of Polystyrene Particles of Various Degrees of Crosslinking and Styrene Monomer Droplets. Combust. Sci. Technol. 1994, 103, 63-84. (6) Panagiotou, T.; Levendis, Y. A. Observations on the Combustion of Pulverized PVC and Polyethylene. Combust. Sci. Technol. 1996, 112, 117-140. (7) Panagiotou, T.; Levendis, Y. A. Observations on the Combustion of Polymers (Plastics): From Single Particles to Groups of Particles. Combust. Sci. Technol. 1998, 137, 121-148. (8) Turi, E. Thermal Analysis; Brandrup and Immergut: Berlin, 1966. (9) Williams, P. T. Waste Treatment and Disposal; Wiley: Chichester, U.K., 1998. (10) Madorsky, S. L. Thermal Degradation of Organic Polymers; Interscience Publishers: New York, 1964.
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Received for review June 9, 2003 Revised manuscript received September 24, 2003 Accepted September 29, 2003 IE030477U
