Effect of Porous and Nonporous Carbonaceous Substrates on

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Energy & Fuels 1999, 13, 984-991

Effect of Porous and Nonporous Carbonaceous Substrates on Polystyrene Thermal Degradation during Fast CO2 Laser Heating Ashish Tripathi,† Chris L. Vaughn,† Waleed Maswadeh,‡ and Henk L. C. Meuzelaar*,† Center for Microanalysis and Reaction Chemistry, The University of Utah, Salt Lake City, Utah 84112, and 2-Geo-Centers, E3220 Aberdeen Proving Grounds, Maryland 21010 Received November 17, 1998

To investigate the effect of porosity on elution of volatiles from a devolatilizing coal particle under pulverized coal combustion (PCC) type heating rate conditions, polystyrene was doped into porous model char (Spherocarb) particles and also coated on the surface of nonporous model char (Glassy Carbon) particles. Particles of approximately 80 µm (( 10 µm) diameter were then individually handpicked. These 80 µm diameter particles were heated to a temperature in the range of 1200-2000 K in 32 ms by means of two converging CO2 laser beams. The eluted products were analyzed by combined gas chromatography/mass spectrometry (GC/MS). The evolved product information was used to construct yield curves. These yield curves were compared to a simple first-order rate law prediction. It was observed that while the styrene yield profile was predicted satisfactorily in the case of nonporous Glassy Carbon, styrene evolution rates were approximately four times slower than predicted in the case of porous Spherocarb. Also, the ratio of secondary pyrolysis products of polystyrene (benzene, toluene, etc.) to a primary pyrolysis product (styrene) was approximately four times higher in the case of Spherocarb than in the Glassy Carbon case. Both findings strongly suggest the presence of transport limitations in porous Spherocarb under PCC-type heating rate conditions.

Introduction The focus of this study is to investigate the effect of porosity on the release of products from small carbonaceous particles (40-100 µm diameter) at the very high heating rates (approximately 105 K/s) characteristic of PCC conditions.1 To achieve these conditions, we constructed and tested the CO2 laser pyrolysis system (shown in Figure 1).2-5 Porous (Spherocarb) and nonporous (Glassy Carbon) spherical particles were selected as the carbonaceous substrates, and polystyrene was chosen as the model polymer for these studies. Not only is polystyrene one of the most extensively studied and characterized polymers,6-22 but also it has some degree * Corresponding author. † The University of Utah. ‡ Aberdeen Proving Grounds. (1) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; Plenum Press: New York, 1985. (2) Maswadeh, W.; Arnold, N. S.; McClennen, W. S.; Tripathi, A.; DuBow, J.; Meuzelaar, H. L. C. Energy Fuels 1993, 7, 1006. (3) Maswadeh, W.; Tripathi, A.; Arnold, N. S.; DuBow, J.; Meuzelaar, H. L. C. J. Anal. Appl. Pyrolysis 1994, 28, 55. (4) Maswadeh, W., Ph.D. Thesis, 1995. (5) Tripathi, A., Ph.D. Thesis, 1997. (6) Tanaka, M.; Shimono, T.; Yabuki, Y.; Shono, T. J. Anal. Appl. Pyrolysis 1980, 2, 207. (7) Urbas, E.; Kaljurand, M.; Ku¨llik, E. J. Anal. Appl. Pyrolysis 1980, 3, 213. (8) Bouster, C.; Vermande, P.; Vernon, J. J. Anal. Appl. Pyrolysis 297, 1980, (9) Sousa Pessoa De Amorim, M. T.; Bouster, C.; Vermande, P.; Vernon, J.; J. Anal. Appl. Pyrolysis 1981, 19. (10) Trojer, L. J. Anal. Appl. Pyrolysis 1981, 2, 353. (11) Toh, H. K.; Funt, B. L. J. Appl. Polym. Sci. 1982, 27, 4171.

of structural resemblance to coal in that alkylaromatic “tar” components are produced upon pyrolysis.23-28 Hence, studying the thermal degradation behavior of polystyrene in the presence of porous and nonporous carbonaceous substrates could help shed light on the effect of porosity on the elution of similar products from coal. (12) Brauman, S. K.; Chen, I. J.; Matzinger, D. P. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 1831. (13) Ide, S.; Nanabu, H.; Kuroki, T.; Ikemura, T. J. Anal. Appl. Pyrolysis 1984, 6, 69. (14) Szekely, T.; Varhegyi, G.; Till, F.; Szabo, P.; Jakab, E. J. Anal. Appl. Pyrolysis 1987, 11, 83. (15) Dean, L.; Groves, S.; Hancox, R.; Lamb, G.; Lehrle, R. S. Polym. Degrad. Stab. 1989, 25, 143. (16) Bouster, C.; Vermande, P.; Vernon, J. J. Anal. Appl. Pyrolysis 1989, 15, 249. (17) Atkinson, D. J.; Lehrle, R. S. J. Anal. Appl. Pyrolysis 1991, 19, 319. (18) Gardener, P.; Lehrle, R. S. Eur. Polym. J. 1993, 29, 2/3, 425. (19) Still, R. H.; Peters, O. A. J. Appl. Polym. Sci. 1993, 50, 989. (20) Lehrle, R. S.; Atkinson, D.; Cook, S.; Gardner, P.; Groves, S.; Hancox, R.; Lamb, G. Polym. Degrad. Stab. 1993, 42, 281. (21) Page`s, P.; Carrasco, F. J. Appl. Polym. Sci. 1996, 61, 187. (22) Hancox, R. N.; Lamb, G. D.; Lehrle, R. S. J. Anal. Appl. Pyrolysis 1991, 19, 333. (23) Solomon, P. R.; King, H. H. Fuel 1984, 63, 1302. (24) Nip, M.; DeLeeuw, J. W.; Crelling, J. C. Energy Fuels 1992, 6, 125. (25) Winans, R. E.; Meuzelaar, H. L. C. Advances in Coal Spectroscopy; Plenum Press: New York, 1992. (26) Greenwood, P. F.; Zhang, E.; Vastola, F. J.; Hatcher, P. G. Anal. Chem. 1993, 65, 1937. (27) Metcalf, G. S.; Windig, W.; Hill, G. R.; Meuzelaar, H. L. C. Int. J. Coal Geol. 1987, 7, 245. (28) Meuzelaar, H. L. C.; Harper, A. M.; Hill, G. R.; Given, P. R. Fuel 1984, 63, 640.

10.1021/ef980254u CCC: $18.00 © 1999 American Chemical Society Published on Web 07/01/1999

Carbonaceous Substrates and Polystyrene Thermal Degradation

Energy & Fuels, Vol. 13, No. 5, 1999 985

Figure 1. Schematic layout of the laser pyrolysis system.

Figure 2. Top view of the laser focusing arrangement.

A polystyrene solution in dichloromethane was coated onto the surface of Glassy Carbon (nonporous) particles and absorbed into Spherocarb (porous) particles. After evaporation of the solvent these particles were heated to final temperatures of about 1400-1700 K at heating rates of approximately 105 K/s. The evolved products were separated and identified by combined gas chromatography/mass spectrometry (GC/MS), and the evolution rates were determined along with the product distributions. Since both types of particles consist almost entirely of carbon, with only trace amount of other elements, the degree of porosity is the primary difference between the two. Therefore, this comparison may help determine the effect of porosity on the production of alkylaromatic product evolution and their apparent evolution kinetics. Experimental Section CO2 laser pyrolysis in conjunction with GC/MS and twocolor micro-pyrometry was used to determine styrene evolution rates from thermally degrading polystyrene. The overall layout

of this system is shown in Figure 1. The various aspects of this laser pyrolysis system are described elsewhere.2-5 A brief discussion follows. CO2 Laser and Optics. The CO2 laser (Apollo, model 3050 OEM) is electronically triggered to produce a pulse of precise duration. The 8 mm diameter laser beam is 47.5% reflected and 52.5% transmitted by a beam splitter. Both the reflected and transmitted beams are individually focused at the center of the cell using high-power mirror objectives (Spawr Corp. model FM90). On basis of the luminous laser beam footprints on quartz wafers,3,4 the laser beam waist can be estimated to be approximately 350 µm in diameter, and delivered laser energy fluxes in the 4 to 80 MW/m2 range. Two very low-power HeNe laser beams (Uniphase, model 1508-0, output 0.95 mW) are positioned such that their beams are coaxial with the CO2 laser beams. This facilitates visual positioning of the particle at the point of intersection of the two laser beams. The energy output of the CO2 laser beam pulse is measured using an integrating beam energy meter (Scientech AC50HD) which receives about 10% of the emitted laser power radiation. Figure 2 illustrates the laser focusing arrangement. Two-Wavelength Radiation Pyrometer. To measure the surface temperature history of the laser-heated particle, a two-

986 Energy & Fuels, Vol. 13, No. 5, 1999

Tripathi et al.

Figure 3. Schematic setup of the two-color micro-pyrometer, the video microscope, and laser pyrolysis stage. color radiation micro-pyrometry subsystem was constructed. The construction, theory, choice of wavelengths, computational techniques, testing, and calibration methods used are described in detail elsewhere.3,4,5 Figure 3 presents the schematic layout of the two-color pyrometer. Radiation emitted from the heated particle is concentrated by a Cassegrainian-type reflective objective (Ealing Optics × 15) and mechanically chopped at frequencies up to 3 kHz (Stanford Research Systems, chopper, model SR540). The chopped radiation beam is split by a 5.065-5.364 µm wavelength band-pass filter (Optical Filter Corp.). The radiation transmitted by this filter is focused on a liquid N2 cooled InSb-type IR detector (Barnes Engineering, model DO4EJ) using a second Cassegrainian-type reflective objective. The remaining radiation is reflected, filtered through a 1.811-2.11 µm wavelength band-pass filter (Optical Filter Corp.), and focused on a second liquid nitrogen cooled InSb IR detector (Barnes Engineering, model DO4EJ) using a third Cassegrainian-type reflective objective. The two Dewarmounted, liquid nitrogen cooled IR detectors send their output to a pair of specially built, low-noise current amplifiers (amplification factor approximately 106). The amplified signal is acquired by a computer via a high-speed, 16 bit resolution A/D conversion board (HSDAS-16, Analogic Inc.) capable of acquiring data at 200 kHz, with the help of SnapShot software (HEM Data Corporation). The entire signal acquisition and amplification process was tested to be fast enough to support a chopper frequency of up to 2.5 kHz. Signal chopping is used to help differentiate the signal from baseline drift and noise.

The amplified, chopped signal acquired from the two detectors is processed through a FFT (fast Fourier transform) routine to produce the corresponding “unchopped” time-domain signal. The calibration curve of the two-color pyrometer is used to provide the corresponding temperature reading from the ratio of the two signals. A mechanical flip mirror in the optical path of the radiation is used to divert the image of the particle on to a sensitive, remote-head CCD camera (Cohu 6400 Series, minimum detectable light level 0.0125 lux). This image is used to visually position the particle within the center of the converging laser beams. Vapor Sampling Inlet-Gas Chromatography/Mass Spectrometry System. To analyze the evolved volatiles during depolymerization, a method for collecting these volatiles and efficiently depositing them on the GC column is essential. Modifying the design described by Arnold et al.,29 an ambient vapor sampling (AVS) system based on a two-tube configuration was designed and constructed.5 The AVS inlet is kept at 140 °C (design limit temperature). Once the eluted products are deposited inside the GC column (DB-1, 0.180 mm bore, J&W Scientific, 2 m long) the resistance heating of the steel jacket on GC column is manually turned on (maximum temperature 180 °C). The products are sequentially desorbed from the column (depending on their partition coefficients) into (29) Arnold, N. S.; McClennen, W. H.; Tripathi, A.; Meuzelaar, H. L. C. Anal. Chem. 1991, 63, 229.



Figure 5. Polystyrene (PS) weight loss (a) and rate curves (b) from three media. These TG data were obtained at a 9 K/min linear heating rate from 300 to 1300 K in helium atmosphere. Glassy Carbon Particles. A 0.5 g aliquot of -170 + 230 mesh screen fraction of Glassy Carbon (Type 2, Aesar, stock # 38014, “zero” surface porosity) was suspended in polystyrene solution (0.01 g/mL). The particles were then removed, spread on a Teflon sheet as a mono-particle layer and left to dry for 1 day in a nitrogen atmosphere at ambient temperature. The dry particles were then collected (they did not stick together) and weighed. The weight of these particles was found to be 0.52 g corresponding to a four percent (by weight) polystyrene content. Figure 4b shows such a polystyrene-coated Glassy Carbon particle (80 µm diameter) on an electron microscopy grid. Figure 4. (a) Spherocarb particle (approx. 80 µm diameter) doped with polystyrene placed on an electron microscopy grid (grid spacing is 50 µm). (b) Glassy Carbon particle (approx. 80 µm diameter) doped with polystyrene placed on an electron microscopy grid (grid spacing is 50 µm). an ion trap mass spectrometer (Finnigan Mat ITD 700) where they are identified. Sample Preparation. Polystyrene was deposited on the carbonaceous substrate from dichloromethane solutions. Spherocarb Particles. A polystyrene polymer (MW 280,000, Catalog # 039A, Scientific Polymers Products) solution in dichloromethane (0.05 g/mL) was prepared. A 4 mL volume of this solution was left to equilibrate with 0.4 g of -170 + 230 mesh screen fraction of Spherocarb (Analabs, 0.6 ( 0.1 porosity) particles. Spherocarb particles were suspended in the polystyrene solution for 3 days with occasional shaking of the suspension. After 3 days the particles were separated from the solution. The wet particles were then spread in a mono-particle layer on a Teflon sheet. The particles were left to dry for a day in nitrogen and washed briefly with dichloromethane to remove the film of polystyrene binding the particles together. The particles were then dried in a nitrogen atmosphere at ambient temperature to dry for 2 days. The final particle sample weighed 0.51 g corresponding to polystyrene content of 22% (w/w). Figure 4a shows such one of these Spherocarb particle (80 µm diameter) on an electron microscopy grid.

Results and Discussion TG/MS Analysis. To characterize Spherocarb particles doped with polystyrene and Glassy Carbon particles coated with polystyrene, we used a special thermogravimetry/mass spectrometry (TG/MS) system described in detail elsewhere.4,30 The TG unit is a Perkin-Elmer TGS-2 balance equipped with a seven series hightemperature furnace. This TG module is coupled to an Extrel quadrupole MS module to measure time-resolved mass spectra in the mass-to-charge ratio (m/z) 10-500 range. This system uses helium at atmospheric pressure as a carrier gas. The TG module was temperature programmed at a 9 K/min heating rate over a 300-1000 K temperature range. Figure 5a shows the weight-loss curves of the two samples and also a thin film of polystyrene prepared by spreading the polystyrene solution described earlier on a Teflon sheet, with subsequent room-temperature drying in nitrogen atmosphere. Figure 5b illustrates the rates observed for the three samples. The polystyrene-coated Glassy Carbon particles show a four percent weight loss and polystyrene(30) Nie, X.; Maswadeh, W.; Tripathi, A.; Meuzelaar, H. L. C. ACS Preprints, Div. Fuel Chem. 1994, 39, 2, 558.


Figure 6. Comparison of the conversion curves of thin polystyrene film decomposition based on styrene yield (dashed lines) and weight loss (solid lines).

doped Spherocarb particles show an 18% weight loss. It was observed that Tmax (temperature at which the maximum weight loss and/or evolution rate is observed) for polystyrene decomposition from thin film, Spherocarb, and Glassy Carbon is 744, 728, and 746 K, respectively. Though these values are very close to each other, the slighly lower Tmax of polystyrene decomposition from Spherocarb could concernably be an effect of catalytic activity of the particle itself. Work done by Ide et al.13 shows a similar trend while degrading polystyrene with active charcoal as a catalyst. Styrene evolution from thermally degrading polystyrene has been used to represent the decomposition process by many researchers.7,14,17,18,21 Using the TG/MS data obtained on thin polystyrene film, we compared the conversion calculated from monomer yield (m/z 104, representing styrene) to the yields obtained from the weight-loss curve. Figure 6 shows the conversion curves based on styrene yield and weight loss. Both curves match each other very well. Also, styrene is known to be the most abundant product of polystyrene thermal degradation under most experimental conditions.6,9,12-18 These observations lend support to our proposed used of the styrene evolution profile to represent polystyrene thermal degradation. The thermal decomposition kinetics of polystyrene are well studied.7-9,11,14,15,17-19,21 In the literature, the reported apparent activation energies of decomposition range from 20 to 83 kcal/mol.7,8,21 Using the data obtained by TG/MS an Arrhenius plot was constructed using a first-order rate law (the plot is shown in Figure 7). The calculated values of activation energies and frequency factors are shown in Table 1. Laser Pyrolysis. Styrene Yield Calibration. The m/z 104 peak area, representing styrene (when measured within the approximate GC retention time window), recorded during thermal degradation of polystyrene can be calibrated to represent the mass of polystyrene present in the original sample. Polystyrene in microsphere form (20.3 µm diameter, 0.32 µm standard deviation) was obtained, courtesy of Bang’s Laboratories

Tripathi et al.

Figure 7. Arrhenius plot of polystyrene thermal decomposition, based on polystyrene decomposing from Spherocarb (weight loss) and polystyrene thin film (weight loss and styrene yield) using TG/MS at 9 K/min linear heating rate. Note that Glassy Carbon data were not included because of the low weight loss. Table 1: Activation Energies and Frequency Factors Calculated from the Arrhenius Plot

case Spherocarb doped with polystyrene (weight loss) thin polystyrene film (weight loss) thin polystyrene film (styrene evolution) used for first-order decomposition kinetics modeling

activation energy (kcal/mole)

frequency factor (1/s)

64

8.34 × 1016

77

2.56 × 1021

72

1.18 × 1019

72

1.18 × 1019

(inventory number L950525C). A known number of these micro-spheres (1,2,3, or 4) were carefully deposited on a single 120 µm Spherocarb particle placed on an electron microscopy grid. This Spherocarb particle was then irradiated with a 50 millisecond (this duration is long enough to ensure complete pyrolysis of polystyrene) CO2 laser pulse. The evolved products were captured by the AVS inlet and analyzed with the GC/MS system. The m/z 104 Peak area (styrene) so obtained is calibrated to the weight of the polystyrene (determined by knowing the volume of the polystyrene micro-sphere and the density of the polystyrene). The calibration curve is shown in Figure 8. Styrene Yield Measurements. Single polystyrene-laden particles (Spherocarb and Glassy Carbon) were first irradiated by a laser pulse of predetermined duration (4, 8, 12, 16, 20, 24, 28, or 32 ms). The evolved products were subsequently captured with the AVS inlet, separated by the GC column, and detected by ITMS. The same procedure was repeated by irradiating the same particle for the second time by a laser pulse of 50 ms ensuring complete thermal degradation of polystyrene. The above procedure was repeated four to six times for each first-time duration, using a different particle. The relative styrene yield was defined as the ratio of styrene


Figure 8. Calibration curve obtained by laser decomposition of known numbers of polystyrene microspheres placed on Spherocarb particles and the styrene obtained. Location of Glassy Carbon and Spherocarb styrene signal and equivalent number of polystyrene microspheres (obtained from weightloss data particle size, and densities of polystyrene and the carbonaceous particles). Error bars represent 95% confidence levels. The filled triangles indicate the comparison of m/z 104 peak area obtained from laser pyrolysis of polystyrene-coated Glassy Carbon and polystyrene-doped Spherocarb particles with equivalent polystyrene microspheres (estimated from weight loss and density data).

Figure 9. Definition of styrene yield. Regions I and II represent reacted and unreacted polystyrene, respectively.

yield (peak area of m/z 104 fragment) released during the first laser irradiation of duration “t ms” to the total


Figure 10. Measured particle surface temperature histories with average and ( 2 times standard deviation error boundaries for Glassy Carbon and Spherocarb. These measurements were obtained from the two-color pyrometer.

styrene released yield during the first and second laser irradiation. Figure 9 parts a and b shows the graphical representation of styrene yield for Glassy Carbon and Spherocarb. Regions I and II show the styrene released during the first and second shot, respectively. In effect, region I can be interpreted as the styrene evolved during the first laser irradiation and region II as the styrene yet to be evolved after the first laser irradiation. Notice that the total styrene (m/z 104) peak area (shown by circles) remains more or less constant (within the limits of particle-to-particle variability), indicating that the total yield is unaffected by the duration of the first laser shot as would be expected from a first order reaction. Polystyrene/Glassy Carbon System. The procedure described in the above section was carried out with simultaneous measurements of the surface-temperature histories of the Glassy Carbon particles. Figure 10a shows recorded temperature histories. The evolved products were analyzed with the GC/MS system. Figure 11a shows a typical total ion chromatogram obtained by irradiating an 80 µm diameter Glassy Carbon particle with a 32 ms CO2 laser pulse (baseline magnified in intensity by 16 times). The average profile of all the summed mass spectra obtained from each particle is shown in Figure 12a. Obviously, we did not observe most of the high-boiling components (such as the dimers, trimers, etc.). This is thought be a result of the relatively low temperature of the AVS inlet (at 140 °C), and the GC column (180 °C maximum). Therefore, we are unable to comment on the formation of high-boiling components. The time-resolved styrene yield curve was constructed by comparing the styrene peak area (m/z 104) obtained from the first laser shot to the total area obtained over both shots. These styrene yields were compared with yield predictions from a first-order decomposition kinetics model, using 72 kcal/mol as the


Figure 11. Examples of total ion chromatograms obtained from polystyrene pyrolysis by irradiating 80 µm diameter Glassy Carbon (styrene peak intensity is 400 arbitrary units) and Spherocarb (styrene peak intensity is 300 arbitrary units) particles loaded with 4% and 22% polystyrene, respectively, with 32 ms CO2 laser pulses.

Figure 12. Average of all the sum spectra obtained from the two carbonaceous media.

activation energy and 1.18 × 1019 s-1 as the frequency factor, as well as the experimentally determined average temperature history and its corresponding temperature error bars. Figure 13a shows these comparisons. The first-order kinetics predicts the observed styrene yields quite well. The thickness of the polystyrene film on Glassy Carbon particles is estimated to be 0.75 µm, equivalent to a total average sample weight of 13.2 ng (based on the assumption that polystyrene is evenly

Tripathi et al.

Figure 13. Comparison of measured styrene yields with firstorder decomposition kinetics predictions using the average temperature history with ( 2 × s.d (illustrated in Figure 9). Notice that styrene yield rates from Glassy Carbon match closely with predicted rates, whereas those from Spherocarb are approximately 4 times slower than predicted.

coated on each particle). This sample size is less than the 0.5 µg critical sample weight suggested by Hancox and Lehrle et al.22 This indicates little or no transport limitation on the pyrolytic reactions, and implies that secondary reactions occurring due to higher residence time of the primary product in the polymer melt are practically nonexistent.22 This is also supported by examining product ratios. Compared to the styrene peak (m/z 104), the relative peak areas of toluene (m/z 91), benzene (m/z 78), and C2-benzenes (m/z 106), all known volatile secondary pyrolysis products,9 are never more than 3% of the styrene peak area, as can be seen in Figure 14a. Polystyrene/Spherocarb System. Repeating the procedure described in the previous section, the GC/MS data along with the temperature history were measured for every Spherocarb particle analyzed. Figure 10b shows all the temperature histories recorded. The total ion chromatogram of eluted products from pyrolysis of polystyrene from a 80 µm diameter Spherocarb particle with a 32 ms CO2 laser pulse is shown in Figure 11b. The average of all the summed spectra obtained from each particle is shown in Figure 12b. Using the same procedure followed for the Glassy Carbon/polystyrene system, a time-resolved styrene yield curve was obtained. This was compared to the first-order decomposition kinetics predictions using the average temperature history and the corresponding error bar as shown in Figure 13b. The first-order kinetics fail to predict the observed styrene evolution. Observed styrene evolution is about four times slower than predicted. The average weight of the polystyrene in these Spherocarb particles is estimated to be 46 ng (from the weight loss and density data). This quantity is thought to be distributed



from examining product ratios. Compared to the styrene (m/z 104) peak area, the relative peak areas of toluene (m/z 91), benzene (m/z 78), and C2-benzenes (m/z 106), and methyl styrene (m/z 117), all known secondary pyrolysis products of polystyrene9 are about 0.12, as observed in Figure 14b. These ratios are about four times higher than those observed for the Glassy Carbon/ polystyrene system. Polystyrene Micro-spheres. The polystyrene microspheres (20.3 µm diameter), each weighing 4.7 ng, used for calibration also provide product distribution information. Since each of these spheres degrades independently, the product ratios should be independent of the number of micro-spheres pyrolyzed. Figure 15 shows that the ratio of major secondary pyrolysis products (such as benzene, toluene, and C2-benzenes) to styrene is independent of the number of micro-spheres. Another point to be noted here is that the product ratios are higher than those observed in Glassy Carbon and lower than those in Spherocarb. This is in agreement with the proposed effect of sample thickness by Hancox and Lehrle et al.22 Conclusions Figure 14. Ratios of secondary products to styrene. Notice that these ratios are about 4 times higher for Spherocarb than for Glassy Carbon.

Figure 15. Ratios of secondary pyrolysis products to styrene obtained from pyrolyzing polystyrene microspheres.

throughout the particle matrix. Although the total sample size is well below the 0.5 µg critical sample weight suggested by Hancox and Lehrle et al.,22 the effective sample thickness could still be several microns. The observed slower rates could be due to the effect of transport limitations on the evolution of the degradation products. This means that secondary reactions may occur, due to the higher residence time of the primary product within the particles.10 This is further evident

The styrene yield rates produced by thermal decomposition of polystyrene thinly coated (0.75 µm) on Glassy Carbon spheres are well predicted by a first-order decomposition model. These rates are approximately four times faster that those from pyrolytic decomposition of polystyrene-loaded (22% w/w) Spherocarb particles. Also the ratios of secondary pyrolysis products (such as benzene, toluene, and C2-benzenes) to styrene are observed to be 4 times higher in the case of Spherocarb than observed from Glassy Carbon particles. These observations are thought to indicate higher residence times of the primary products in Spherocarb (porous) when compared to Glassy Carbon (nonporous). The longer residence time in the porous particles could be attributed to the mass transport limitation in the form of pore diffusion. The pore diffusion resistance reduces the rate of styrene release, increasing the residence time. This results in a higher probability of secondary pyrolysis product formation. On the basis of these observations, pore diffusion-limited tar evolution rates may be expected to exist under pulverized coal combustion conditions, where porous carbonaceous particles in the same overall size range are heated to furnace temperatures in the 1400-1800 K range at heating rates up to 105 K/s. Acknowledgment. We are thankful for the helpful discussions we had with Dr. Thomas H. Fletcher, Dr. Ronald J. Pugmire, Dr. Philip J. Smith, Dr. Alva D. Bear, Dr. William H. Mclennen, and Mr. Neil Arnold. This work was funded by the Advanced Combustion Engineering and Research Center (ACERC). Funds for this center are provided by NSF, state of Utah, 23 industrial participants, and the U.S. DOE. EF980254U

Effect of Porous and Nonporous Carbonaceous Substrates on

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