Polycyclic Aromatic Hydrocarbon (PAH) and Soot Formation in the

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Polycyclic Aromatic Hydrocarbon (PAH) and Soot Formation in the Pyrolysis of Acetylene and Ethylene: Effect of the Reaction Temperature Nazly E. Sánchez,* Alicia Callejas, Á ngela Millera, Rafael Bilbao, and María U. Alzueta* Aragón Institute of Engineering Research (I3A), University of Zaragoza, Campus Río Ebro, 50018 Zaragoza, Spain ABSTRACT: The formation of soot and polycyclic aromatic hydrocarbons (PAHs) has been studied during the pyrolysis of acetylene and ethylene at different reaction temperatures (1073−1423 K) in a tubular flow reactor at atmospheric pressure. The 16 PAHs classified by the United States Environmental Protection Agency (U.S. EPA) as priority compounds, together with light gases present at the outlet gas stream, were quantified by a chromatographic method. Soot formed was collected on a filter at the reactor outlet and later quantified. In this way, the relationship between PAH, gas, and soot formation can be discussed. The distribution of the target PAHs in the different phases (at the gas phase, adsorbed on soot, and/or sticked on reactor walls) is also analyzed. The speciation of the individual PAH compounds was achieved by a combination of Soxhlet extraction, extract concentration by a rotary evaporator, and gas chromatography coupled to mass spectrometry (GC−MS). The present study shows that, in the pyrolysis of both acetylene and ethylene, while soot formation is enhanced by increasing the temperature, the PAH yield exhibits a maximum in the evaluated temperature range, and such a maximum value depends upon the hydrocarbon used (ethylene or acetylene). However, the PAH distribution in the different phases does not seem to be influenced by hydrocarbon used. PAHs from ethylene and acetylene pyrolysis are seen to be mainly adsorbed on soot preferably than on other places, except for naphthalene (NAPH) in the pyrolysis of ethylene, in which case a higher concentration was found at the gas phase. 1073 to 1223 K,14 while soot formation is favored at higher temperatures.15 These results indicated a clear effect of the reaction temperature on pyrolysis compounds. Murphy et al.7 showed that the gas composition in pyrolysis processes effectively changes as the temperature changes, and such a trend also depends upon the hydrocarbon used. Recently, Norinaga et al.16 studied in detail the pyrolysates from ethylene, acetylene, and propylene to find intermediates that are believed to be crucial in PAH formation. Their experimental results, obtained in a vertical plug flow reactor, showed that all PAHs evaluated are also influenced by the reaction temperature and hydrocarbon used. Mathieu et al.17,18 determined that the mass range of PAHs detected on soot depends upon the reaction temperature as well as hydrocarbon used. These PAHs together with those found at the gas phase can give information about the main compounds involved in the surface growth of soot. Additional works, considering this issue, include studies on flames of ethane, ethylene, acetylene, and other hydrocarbons.19−21 However, to our knowledge, there is not any study in the literature that addresses the formation of both soot and PAHs and the distribution of PAHs between the gas and soot surface in the pyrolysis of their main precursor (C2H2 and C2H4) using tubular flow reactors. Several other studies have been focused on kinetic modeling for combustion of light hydrocarbons and growth of PAHs, for achieving the prediction of the polyaromatic formation in the

1. INTRODUCTION The increasing interest for the knowledge of polycyclic aromatic hydrocarbon (PAH) formation is due to the health problem concerns attributed to them1,2 and their role as important soot precursors. The formation of PAHs in fuel-rich regions is often mentioned as being the first step toward the formation of soot particles.3 The close relation between PAHs and soot generates moreover undesired effects, because the known carcinogenic and mutagenic effects of soot particles are caused by the direct association of the PAHs adsorbed on their surface.4−6 It is accepted that ethylene and acetylene are important soot precursors.7 These compounds together with methane may be formed in different thermochemical processes of carbonaceous materials.8−10 However, methane and natural gas conversion would require a higher temperature than acetylene and ethylene conversion. Acetylene and ethylene can undergo a process of simultaneous dehydrogenation at the gas phase to form PAHs and soot according to the hydrogen abstraction/C2H2 addition (HACA) route.11 Thus, conversion of acetylene and ethylene can play a crucial role in explaining the formation of aromatic and polyaromatic species. Despite significant progress made in both the experimental studies and theoretical interpretation of soot and PAH formation processes, some aspects of these complex processes still remain poorly understood.12,13 Such is the case of the effect of different main soot precursors on both PAH and soot formation and their interrelation in pyrolysis processes, which are subject of controversy. Studies have shown that under pyrolysis conditions using acetylene as a direct PAH−soot precursor, a considerable amount of PAHs is formed under temperature conditions from © 2012 American Chemical Society

Received: May 3, 2012 Revised: July 4, 2012 Published: July 6, 2012 4823

dx.doi.org/10.1021/ef300749q | Energy Fuels 2012, 26, 4823−4829

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different thermochemical processes,22,23 with most of them only from modeling points of view. Nowadays, more detailed studies are needed to fill the gaps between experimental and numerical results. In this context, the present work reports an experimental study on the influence of the temperature and the hydrocarbon used (C2H2 and C2H4) on PAH and soot formation in pyrolysis processes, using well-controlled laboratory-operating conditions. The possible PAH compounds involved in reaction channels related to the HACA mechanism,16,24,25 in the pyrolysis of acetylene and ethylene, are determined in the present work. They are included in the list of 16 PAHs classified by the United States Environmental Protection Agency (U.S. EPA) as priority pollutants (EPA−PAH).26 These are naphthalene (NAPH), acenaphthylene (ACNY), acenaphthene (ACN), fluorene (FLUO), phenanthrene (PHEN), anthracene (ANTH), fluoranthene (FANTH), pyrene (PYR), benzo(a)anthracene [B(a)A], chrysene (CHR), benzo(b)fluoranthene [B(b)F], benzo(k)fluoranthene [B(k)F], benzo(a)pyrene [B(a)P], indene(1,2,3-cd)pyrene [I(123-cd)P], dibenzo(ah)anthracene [DB(ah)A], and benzo(g,h,i)perylene [B(ghi)P]. The corresponding molecular mass for each EPA−PAH is shown in Table 1. The soot amount and light gases at the exit of the reactor have

gases reach this reactive zone, and it can be calculated as a function of the temperature, tr (s) = 1706/T (K). After this time, the pyrolysates were chilled by the contact surface when they pass through a jacketed quartz tube cooled by compressed air. The pyrolysis installation has been described in detail elsewhere.e.g. 28,29 Soot was collected using a quartz fiber filter with a pore diameter lower than 1 μm, placed at the outlet of the quartz tube. PAHs were collected, after the soot-collecting filter, by trapping them in a thin tube of 300 mm length and an external diameter of 10 mm packed with XAD2 resin, supplied by Supelco. Light hydrocarbons at the gas phase were quantified at the outlet of the resin tube, using an Agilent Technologies gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and HP Plot MoleSieve and HP Plot Q columns and a flame ionization detector (FID) with an HP-PONA column. The calibration was performed using a standard mixture of C2H2, H2, CH4, C2H4, C2H6, C3H8, C3H6, 1,3-butadiene, i-C4H10, n-C4H10, C6H6, C7H8, and C8H10. The total time of every experiment was fixed at 1.5 h, using both acetylene and ethylene with a constant inlet concentration of 30 000 parts per million by volume (ppmv) in a bath of nitrogen. This latter is added to achieve a total flow rate of 1000 [standard temperature and pressure (STP)] mL/min. The time of the experiment (1.5 h) is necessary to collect the adequate amount of soot and PAHs for further processing and analysis. The hydrocarbon pyrolysis took place at atmospheric pressure. Experiments were performed in a temperature interval ranging from 1073 to 1423 K, as shown in Table 2.

Table 1. Molecular Mass for Each EPA−PAH Studied

Table 2. Experimental Conditions of PAH and Soot Formation in the Pyrolysis of 30 000 ppmv of Ethylene and Acetylene

PAH

PAH abbreviation used

molecular mass (g/mol)

naphthalene acenaphthylene acenaphthene fluorene phenanthrene anthracene fluoranthene pyrene benzo(a)anthracene chrysene benzo(b)fluoranthene benzo(k)fluoranthene benzo(a)pyrene indene(1,2,3-cd)pyrene dibenzo(ah)anthracene benzo(g,h,i)perylene

NAPH ACNY ACN FLUO PHEN ANTH FANTH PYR B(a)A CHR B(b)F B(k)F B(a)P I(123-cd)P DB(ah)A B(ghi)P

128 152 154 166 178 178 202 202 228 228 252 252 252 276 278 276

also been determined. Thus, the present results are valuable to evaluate possible mechanisms, which involve not only reactions at the gas phase but also soot formation. Considering that PAHs can be distributed on different surfaces (adsorbed on soot and sticked at the reactor walls) and at the outlet gas stream, mainly depending upon their molecular weights27 and vapor pressures but also the environmental temperature, pressure, PAH concentration, and soot characteristics, the collection and analysis methods used for PAH determination were developed taking into account these aspects.

set

source of carbon

reaction temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

ethylene

1073 1123 1173 1223 1273 1323 1373 1423 1073 1123 1173 1223 1273 1323

acetylene

Mainly dependent upon their molecular weights and vapor pressures, PAHs can be distributed between the gas phase or condensed on solid surfaces, such as the soot particles and reactor walls. In general, while the most volatile compounds are released at the gas phase, compounds containing three or more aromatic rings in their structure are associated with particulate matter emission.30 Considering this, we have optimized a collection method and an analytical method developed for the characterization and quantitation of total EPA−PAH formed.31 Lighter PAHs at the gas phase were collected using a tube with 5 g of XAD-2 resin. A total of 3 g was placed in the first part of the tube for collecting the PAHs, and with separation by quartz wool, the rest of the resin was used as a blank to ensure that all PAHs were previously adsorbed in the first part. Both fractions were analyzed separately. The XAD-2 resin was selected because of advantages shown in previous studies.31,32 The heaviest PAHs, which appear adsorbed on either soot or the reactor walls, were collected by means of both the soot collection system and washing the reactor with 100 mL of dichloromethane, respectively. The PAH analyses were carried out immediately after each experiment, to avoid any loss of compounds during storage. The samples collected underwent Soxhlet extraction in accordance with the 3540C U.S. EPA method.33 The extraction parameters were

2. EXPERIMENTAL SECTION Pyrolysis experiments were carried out in a quartz tubular flow reactor of 45 mm inside diameter and 800 mm in length, linked to a soot and PAH collection system and a gas analysis system. It is worth mentioning that the quartz reactor was cleaned with fluorhydric acid (HF) to avoid the catalyzed reactions for reactor walls. The longitudinal temperature profile inside the reactor was determined by means of a S-type fine-wire thermocouple. An isothermal reaction zone (±25 K) of 6 cm was found. Thus, the gas residence time was defined from the moment in which 4824


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Figure 1. Temperature influence on yield profile of different products in the pyrolysis of (a) ethylene and (b) acetylene. explained in a previous work.31 The recommendations of U.S. EPA method 8270D have been followed.34 The EPA−PAH analysis was performed using a 6890A GC coupled to a MSD 5975C (GC−MS) with a capillary column DB-17Ms (60 m × 0.25 mm inner diameter × 0.25 μm) from Agilent Technologies, virtually equivalent to (50% phenyl)-methylpolysiloxane. A total of 1 μL of sample was injected by 7683B autosampler in splitless mode. The temperature program was as follows: an initial temperature at 353 K held for 15 min, then raised at 5 K/min to 383 K and held for 5 min, a second heating rate of 5 K/min to 563 K held for 35 min, and finally, a third heating rate of 1.5 K/min to 593 K held for 5 min. Helium was used as the carrier gas at a flow rate of 1 (STP) mL/min. The quantitation was achieved in the selected ion monitoring (SIM) mode of the MS to enhance the selectivity and sensitivity of the method using the ion fragments that coincide with the molecular masses presented in Table 1. Because of good resolved peaks, the quantitation using peak area was used, despite the complicated organic matrix, in which PAHs were found. The minimum value of the correlation coefficient (r2) of the calibration curves for the EPA−PAH was 0.986. The response factors were calculated from each calibration curve in accordance with the U.S. EPA 8270D method34 to determine the concentration levels of each PAH. Analysis from XAD-2 resin showed that effectively the greatest quantity of PAHs was adsorbed on the first part of the adsorption tube. Only a very small quantity, which was not observed to be higher than 2% of the total PAHs at the gas phase, was found on the blank. The method was validated using fully characterized soot from a diesel engine (SRM 1650b) and a commercial soot used as diesel soot surrogate called Printex-U. The recoveries of EPA−PAH for the SRM 1650b were higher than 80% in the most of cases. The PAH analyses using Printex-U showed good repeatability of the results. Hence, it is concluded that the method has high reliability to the determination of EPA−PAH. The subsequent PAH analysis included the determination of their weights, using the calibration curves for each PAH priority. This weight was converted into ppmv, by means of the ideal gas equation and the gas total volume in each experiment. The PAH results in the present work are thus given in ppmv or yield percentage.

The acetylene and ethylene concentration at the reactor outlet was also quantified together with other gas products, such as H2, CH4, C2H6, C3H8, C3H6, 1,3-butadiene, i-C4H10, n-C4H10, C6H6, C7H8, and C8H10. The yield of the total gases from ethylene and acetylene pyrolysis is presented in Figure 1, together with the yields to PAHs and soot. The yield is defined as the percentage of the carbon amount in the soot, in PAHs or gases, related to the carbon total amount at the inlet of the reactor. It is worth clarifying that the yield of the gas fraction includes the amount of reactant hydrocarbons (acetylene or ethylene) found at the reactor outlet. To simplify the analysis and achieve a better understanding, the PAH yield is calculated considering the sum of all EPA−PAH, i.e., those found on soot, reactor walls, and XAD-2 resin. Previous studies showed that, for both hydrocarbons (acetylene and ethylene), as the reaction temperature is increased, more conversion of reactants by pyrolysis is produced, accompanied by an increase in the soot yield and a decrease in the gas yield. Moreover, the yield to soot was found to be higher in the acetylene pyrolysis compared to ethylene experiments under similar conditions to those used in the present work.15,29 In those works, the profile of the PAH yield was not reported. The same behavior for soot and gas yields can be observed in Figure 1. In addition, the PAH yield from ethylene pyrolysis (Figure 1a) is seen to exhibit an undefined maximum at temperatures between 1273 and 1323 K, while in the pyrolysis of acetylene, the PAH maximum is found at 1223 K (Figure 1b). This coincides with the results by Marsh et al.,35 who, in their experiments on catechol pyrolysis at different reaction temperatures, using a laminar flow reactor, found that most EPA−PAH showed a peak yield around 1223 K. A maximum for the total concentration of PAHs emitted as a function of the temperature was also observed by Mastral et al.36 in their experiments on coal combustion in a fluidized bed. A maximum concentration for some individual PAHs was also reported by Sharma and Hajaligol37 and Aracil et al.38 in the pyrolysis of polyphenolic compounds and polyvinyl chloride, respectively. The differences between the PAH yield for each hydrocarbon studied (Figure 1), i.e., the temperatures at which the maximum PAH yield is observed, provided evidence for the conversion of C2H4 to C2H2 as a first pyrolysis step. Thus, for the acetylene case, the direct aromatic precursor is available to form PAHs, whereas in the ethylene case, it first reacts to form acetylene, causing the observed behavior. This observation is in agreement with the findings by Murphy et al.,7 who studied the effect of the

3. RESULTS AND DISCUSSION Pyrolysis of 30 000 ppmv of acetylene and ethylene was carried out in a tubular flow reactor at a fixed total flow rate of 1000 (STP) mL/min at atmospheric pressure. The effect of the temperature from 1073 to 1423 K on the concentration of EPA− PAH, lighter gases and the amount of soot was evaluated. The influence of the residence time, under similar experimental conditions, was shown in a previous work.31 4825


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Figure 2. Concentration of the individual total EPA−PAH at different reaction temperatures: (a) 1073 K, (b) 1123 K, (c) 1173 K, (d) 1223 K, (e) 1273 K, and (f) 1323 K.

temperature on the composition of gaseous effluent during pyrolysis of different hydrocarbons. Their results showed that acetylene was the major component of the effluent in ethylene pyrolysis. Then, it was established that acetylene acts as a key precursor on soot and aromatic hydrocarbon formation as the HACA mechanism suggests.

The results of the individual total EPA−PAH concentration obtained from ethylene and acetylene pyrolysis for experiments performed between 1073 and 1323 K are shown in Figure 2. Individual total PAH refers to the sum of each individual PAH found either in the XAD-2 resin (proceeding from the gas phase), adsorbed on soot, or on the reactor walls. 4826


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Figure 3. Individual PAH distribution on the different surfaces and at the gas phase, at the PAH yield maximum temperatures in the pyrolysis of (a) ethylene, 1273 K; (b) ethylene, 1323 K; and (c) acetylene, 1223 K.

present in much smaller quantities in the combustion/pyrolysis of different hydrocarbons. Figure 2 also shows that initially large PAHs increase with the temperature, reaching a maximum concentration at an intermediate temperature. This may indicate, to some extent, the consumption of large PAHs for producing soot, which can be explained because a marked formation of soot begins at these temperatures. This agrees with the information reported by Wang and Cadman,39 who affirm that PAHs (in particular, the large PAHs) must be involved in the formation of soot; thus, their concentrations can be reduced with an increasing soot yield. At temperatures of 1073, 1123, 1173, and 1223 K (panels a−d of Figure 2), a higher concentration for all PAHs formed under acetylene pyrolysis is observed. Individual total PAH concentrations coming from ethylene pyrolysis at reaction temperatures of 1073 and 1123 K were scarcely detected, whereas they exhibited a marked increase in their concentrations for a pyrolysis temperature of 1173 K (Figure 2c). It is highlighted that, as shown in Figure 1, this reaction temperature was not high enough to form soot, which was indeed observed from the pyrolysis temperature of 1223 K. At this latter temperature, a considerable increase in the PAH concentration, mainly including heavier PAHs, from ethylene pyrolysis was noted (Figure 2d).

In Figure 2, it is observed that, under all temperature conditions and for the hydrocarbons evaluated (ethylene and acetylene), the compounds with higher concentrations were NAPH, ACNY, FLUO, PHEN, ANTH, FANTH, and PYR, which belong to compounds with 2−4 aromatic rings or lighter PAHs within the EPA−PAH, except for ACN, which was observed in concentrations close to the detection limit. This fact was previously showed by Wang and Cadman,39 who observed a higher amount of lighter PAHs, especially PYR and FLUO, in the pyrolysis of different hydrocarbons. This also coincides with that observed by Wornat’s group,40−42 who mention that, under their experimental conditions, PAHs of smaller ring numbers are produced in higher yield than those of large ring numbers. It is also observed in Figure 2 that NAPH was the most abundant compound in the hydrocarbon pyrolysis at the three lowest temperatures considered (1073, 1123, and 1173 K). On the opposite side, ACNY was found as the majority compound at the highest temperatures analyzed (1223, 1273, and 1323 K). These latter conditions may favor the conversion of NAPH to ACNY. Higher molecular weight aromatics, including B(a)A, CHR, B(b)F, B(k)F, B(a)P, I(123-cd)P, DB(ah)A, and B(ghi)P, were detected in minor concentrations. This fact is consistent with the conclusions given by Wang and Cadman,39 who showed that large PAHs, with 5 or more aromatic rings in their structure, were 4827


Energy & Fuels Similar concentrations of PAHs from acetylene and ethylene pyrolysis were found at 1273 K (Figure 2e), except for NAPH, which presented a much lower concentration in the pyrolysis of ethylene. Finally, in the most of the cases, the PAHs from ethylene pyrolysis at 1323 K (Figure 2f) increased, even exceeding the concentrations found from acetylene pyrolysis at the same temperature. The results of the distribution of PAHs on the different surfaces and at the gas phase, in the pyrolysis of acetylene and ethylene, have also been analyzed. In Figure 3, the individual PAH distribution, for the temperature corresponding to the maximum in which they showed the highest yield, is presented. With respect to ethylene pyrolysis, with an undefined maximum between 1273 and 1323 K (Figure 1), the results obtained for both temperatures have been plotted in panels a and b of Figure 3. As observed, the PAH distribution in such schemes is similar. In general, a much higher concentration of PAHs was found on soot than in other places, except for the NAPH, which was found mainly adsorbed on resin, corresponding to the gas phase. Only a small concentration of PAHs has been collected from the reactor walls in these conditions. All PAHs from acetylene pyrolysis at 1223 K (Figure 3c) were adsorbed mainly on soot. Only a small fraction of NAPH and ACNY was observed at the gas phase. The reactor walls seem to be an important place where PAHs appear adsorbed. All results also show that the majority of compounds adsorbed on soot were NAPH, ACNY, PHEN, FANTH, and PYR. All of these compounds are important aromatic intermediates, which are involved in the HACA mechanism during the large PAH and soot formation.18,24,43

ACKNOWLEDGMENTS

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REFERENCES

The authors express their gratitude to MICINN and FEDER (Project CTQ2009-12205) for financial support. Nazly E. Sánchez acknowledges Banco Santander Central Hispano, Zaragoza University, and the Colombian Institute for the Development of Science and Technology (COLCIENCIAS) for the predoctoral grant awarded. The authors also thank the research group in waste, pyrolysis, and combustion from the University of Alicante, especially Professor Rafael Font for assistance with developing the analytical method.

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4. CONCLUSION The pyrolysis of ethylene and acetylene under a wide range of temperatures (1073−1423 K) was carried out focusing mainly on EPA−PAH and soot formation. From this study, several conclusions may be drawn. Results show that, while the soot formation is enhanced for both ethylene and acetylene pyrolysis with an increase of the temperature, the PAH yield exhibits a maximum. Moreover, the yield to soot was found to be higher in the acetylene pyrolysis under all cases studied. The major compounds, found in all of the cases studied, were NAPH, ACNY, FLUO, PHEN, ANTH, FANTH, and PYR. Higher molecular weight aromatics, including B(a)A, CHR, B(b)F, B(k)F, B(a)P, I(123-cd)P, DB(ah)A, and B(ghi)P, were detected in small concentrations and adsorbed exclusively on soot and reactor walls. The individual PAH analyses of different samples showed that the PAH concentration adsorbed on soot is considerably higher than that found condensed on the reactor walls or at the gas phase. This is important because it indicates that the analysis of PAH formation in sooting experiments should take into account both the PAHs present at the gas phase and adsorbed on different surfaces (e.g. soot and reactor walls).

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +34976761876. Fax: +34976761879. E-mail: [email protected] (N.E.S.) or [email protected] (M.U.A.). Notes

The authors declare no competing financial interest. 4828


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