Investigation of Pyrolysis of Hydrocarbons and Biomass Model

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Investigation of Pyrolysis of Hydrocarbons and Biomass Model Compounds Using a Micropyrolysis Flow Cell A. B. Ross, A. Lea-Langton, E. M. Fitzpatrick, J. M. Jones, A. Williams,* G. E. Andrews, H. Li, and K. D. Bartle Energy and Resources Research Institute, University of Leeds, Leeds LS2 9JT, United Kingdom ABSTRACT: Hydrocarbon growth mechanisms based on a combination of small hydrocarbon fragments, such as C2/C4, have been postulated in high-temperature flames. However, there is considerable evidence of alternative pathways, especially in lower temperature regimes, in particular, the importance of resonance-stabilized radicals based on cyclopentadienyl (CPDyl) moieties. Further evidence of polycyclic aromatic hydrocarbon (PAH) growth mechanisms can be provided by analytical pyrolysis experiments using high-temperature flow cells. An analytical micropyrolysis flow cell method has been developed, enabling reactive pyrolysis of solid and liquid samples. This uses a SGE microflow cell pyrojector interfaced to a CDS 5200 series pyrolyzer in adsorbent mode for the product concentration, followed by gas chromatographymass spectrometry (GCMS) or Fourier transform infrared (FTIR) analysis. The flow cell system enables residence time and stoichiometry to be varied. The benefits of the system over a filament pyrolyzer are demonstrated. A number of applications of this technique are presented to illustrate the potential of this system to investigate biofuel decomposition, PAH growth mechanisms, and soot formation from different hydrocarbon and oxygenated fuels. Evidence is obtained indicating that resonance-stabilized intermediates, especially CPDyl, are important in PAH growth mechanisms for a wide range of fuel types.

1. INTRODUCTION Thermal pyrolysis or thermal cracking of hydrocarbons is a process widely used in the chemical industry, and there has been extensive knowledge of the underlying scientific basis for some time.1,2 Increasingly, biomass will be used as a feedstock, raising new issues in thermal processing. In parallel with this, there has been a dramatic increase in the knowledge of combustion, especially of complex liquid hydrocarbon fuels, biofuels, and biomass. All combustion-based energy systems require an understanding of the chemistry involved to increase operational efficiency and simultaneously reduce emissions. Most liquid fuels are used in transport, and the pyrosynthesis of polycyclic aromatic hydrocarbon (PAH) emissions from diesel fuel components is a problem area that is complicated by the presence of up to 9% PAHs in the fuel in Europe and more in the U.S.A. Oxygenated biofuels are increasingly added to diesel, and the pyrosynthesis of PAHs from biofuels is investigated in the present work. Combustion systems can be investigated by a comparison of experimentally derived data to kinetic models, generally with model compounds and simple surrogate fuels. The advent of powerful computation procedures has enabled the development of large chemical mechanisms to describe these processes especially for engine fuels.35 Numerous laboratory-scale systems have been developed for obtaining experimental data for combustion and pyrolysis systems, including the use of flow reactors, pyroprobegas chromatographymass spectrometry (GCMS), shock-tube experiments, and detailed flame diagnostic methods. The latter have been particularly rewarding. Hansen et al.6 reviewed flame sampling molecular-beam mass spectroscopy, and concluded that a variety of intermediates, including resonantly stabilized radicals and closed shell intermediates, are important to hydrocarbon and oxygenate combustion. Naik et al.7 modeled blends of alkane r 2011 American Chemical Society

components as surrogates for FischerTropsch and biomassderived jet fuels. This work confirmed that fuel consumption was dominated by abstraction reactions followed by β-scission, breaking larger molecules into smaller hydrocarbons. A review of kinetic mechanisms involving biodiesel was also conducted by Lai et al.8 Under conditions of incomplete combustion, these fuels can give both PAHs and/or oxygenated products. In diesel engines, incomplete combustion is rarely significant today and it is the pyrolysis reactions in the fuel-rich center of the spray at high temperatures that are of main concern. Increased knowledge on the formation of PAHs during combustion is important because they are precursors to soot particles as well as being carcinogenic.916 Soot particles themselves can impact climate and atmospheric chemistry. The combustion of biomass from both domestic heating and forest fires releases a large emission of oxygenated species into the atmosphere, for example, organic acids, ketones, and phenols, as both volatile organic compounds (VOCs) and adsorbed onto the soot particles.11 Conventional reaction pathways have been postulated for the formation of PAHs and soot from hydrocarbon fuels, but less is known of the routes involved in biomass-derived fuels. The mechanism has been extensively studied in flames, where acetylene is the basis of the H-abstractionC2H2-addition (HACA) model of soot formation.9,10,18 PAH is taken to be the main building block for soot formation, and emitted PAHs are essentially byproducts. However, other routes are involved especially at low temperatures, and a number of studies have been made of them.1,1628 Even in non-sooting low-temperature Received: February 2, 2011 Revised: May 20, 2011 Published: May 20, 2011 2945

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Figure 1. Schematic diagram of the pyrolysis reactor interface.

reaction conditions (5001000 °C), the formation of PAH compounds can occur. For example, Norinaga et al.17 investigated a number of different pathways to PAH formation from the pyrolysis of unsaturated light hydrocarbons in a flow reactor in a temperature range of 10731373 K. Comparisons were made between numerical simulations and the experimental results. At high temperatures, pyrolysis of acetylene showed that methyl radicals are important in the formation of cyclopentadienyl (CPDyl), which may lead to larger PAHs.17 Hydrocarbon mechanisms based on the combination of small hydrocarbon fragments, such as C2/C4, have been proposed.9,10 However, there is evidence of alternative pyrolytic and recombination pathways, especially at lower temperatures, in particular, the importance of resonance-stabilized radicals based on CPDyl moieties in some mechanisms.2124 CPDyl radicals can combine to form naphthalene,2224 and indenyl radicals can recombine to produce larger PAHs containing a five-membered ring or exclusively six-membered rings.25,26 Biomass fragments are known to produce both cyclopentadiene (CPD) and indene, and therefore, the influence of these species on the formation of larger aromatics is of interest. There is considerable interest in oxygenated fuels for engine applications, generally in the form of ethanol or biodiesel blended with conventional petroleum fuel. Although the effect of these oxygenates has been investigated in terms of regulated emissions, there is much less data available on the effects on unregulated emissions, including VOCs and PAHs. The combustion of biomass from wood also requires more experimental data regarding these types of species. In the case of both engine biofuels and solid biomass, CPD can be produced from phenoxy radicals,22 which are produced from phenols produced during biofuel or biomass decomposition. CPDyl radicals can recombine to produce a range of PAHs, including naphthalene and indene.25,26 The reaction with naphthalene, CPD, and indene can lead to PAH growth to three- and four-ring PAHs.25 Previous work by the authors has identified CPD, naphthalene, and indene in the pyrolysis products of eugenol and furfuryl alcohol.11 Similarly, Ledesma et al.27 identified these hydrocarbons in the pyrolysis products of catechol, while Lu and Mulholland28 have proposed that the presence of CPDyl moieties results in the formation of PAH products that cannot be represented by the acetylene addition and arylaryl addition pathways. The aromatic growth is initiated by the formation of resonance-stabilized CPDyl and indenyl radicals, whose stability allows for further

rearrangements.28 These mechanisms are investigated using the pyrolysis flow cell technique presented in this paper. All of this work points to the necessity of determining the identity of the products of pyrolytic reactions to complement theoretical studies. This paper presents a flexible technique that can be used for investigation of pyrolysis and reactive pyrolysis reactions using a microflow cell coupled to GCMS. Previously reported pyrolysis and oxidative pyrolysis experiments have been performed in flow cells with subsequent offline capture and analysis of VOCs and PAHs.16 In contrast, the hyphenated pyrolysis system developed for this study enables online trapping and subsequent in situ transfer to GCMS. For many studies of PAH formation that have used flow systems, sampling can be a problem if the samples have to be removed for analysis. The technique used here overcomes this issue with almost immediate direct analysis. The influence of the temperature and residence time on PAH formation was investigated using hexadecane, biodiesel, and the biomass pyrolysis intermediate compounds, furfural, furfuryl alcohol, eugenol, and guaiacol.

2. EXPERIMENTAL SECTION 2.1. Apparatus and Method. The system consists of a modification to the CDS 5200 series pyrolyzer in adsorption mode by interfacing an SGE pyrojector to the injection port of a CDS 5200 series filament pyrolysis unit (CDS Analytical, Inc.). The flow cell reactor system consists of a stainless-steel tube with a total length of 250 mm, 6 mm outer diameter, and 4.5 mm inner diameter, through which a stream of helium carrier gas flows. The overall arrangement is shown in Figure 1. The sample is injected into the first section, which is 50 mm long and heated to about 200 °C. This is followed by a 60 mm long heated reactor section, which forms the main reaction zone and is heated between 600 and 900 °C, as required. After this is a 100 mm section maintained at 200 °C, in which the reaction products are quenched, and this section is inserted into the CDS 5200 interface and absorbent trap. The total flow through the reactor was varied between 10 and 100 mL/min (measured at 20 °C), giving laminar flow through the reactor. The reaction zone was assumed to be given by the length of the heated reactor, and the heating time of the gas stream to the reaction temperature is approximately compensated by the quenching time. Reaction times were calculated on that basis. For liquid samples, 0.5 μL of sample was injected directly into the flow cell using a syringe over a period of 0.5 s. For solid samples, the solid syringe injector (SGE) was used and 1 mg of solid was injected into the 2946

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Energy & Fuels reactor. After the reactor, the resulting products (C4C20) are trapped onto an adsorbent trap (Tenax TA) at 40 °C by operating the CDS pyrolyzer in adsorbent mode. The trap is then desorbed at 300 °C in a flow of He onto the chromatographic column. The gaseous products (H2, CO, CO2, CH4, etc.) are not trapped onto Tenax TA and are vented. The gases can be collected separately and analyzed, but this was not performed in the present experiments. The use of the adsorbent trap enables the sample to be pyrolyzed in a reagent gas, after which valve switching ensures only He carrier gas enters the chromatographic column. The CDS 5200 pyrolysis unit was connected to a Shimadzu 2010 GCMS. The products were separated on a Rtx 1701 60 m capillary column, 0.25 mm inner diameter, 0.25 μm film thickness, using a temperature program of 40 °C and static time of 2 min, ramped to 250 °C at a ramp rate of 4 °C/min with a static time of 30 min; the column head pressure at 40 °C was 2 bar. For all GCMS studies, the chromatograms were assigned on the basis of the National Institute of Standards and Technology (NIST) 2005 Mass Spectral Library database, from previous literature, and by known retention times, as previously described.11 The range of products was considerable, and they were not determined quantitatively in this paper. The influence of reaction conditions is reported on a comparative basis. The products from this system are compared to the pyrolysis products using the CDS 5200 pyrolysis unit in normal operating adsorbent mode using the platinum filament pyrolyzer. In the filament pyrolyzer experiments, the sample (1 mg) is loaded into a quartz boat and heated to the same temperatures (500900 °C) with a heating rate of 20 °C/ms. Additional pyrolysis results were obtained using a flow cell coupled with a Fourier transform infrared (FTIR) detector to provide information on other species and to assist the quantitative estimation of the product yields. The sample (5 μL) is injected into a quartz tubular reactor heated using a tube furnace to 700900 °C. The flow reactor has an internal diameter of 30 mm, a length of 420 mm, and a nitrogen (oxygen-free) gas flow rate of 2 L/min (measured at 20 °C). Samples were removed by a heated (180 °C) sampling line, 1 m long, and measurements were made at the exit using a FTIR detector (Temet Gasmet CR Series FTIR). The residence times are comparable to those used in the experiments with the CDS 5200 pyrolysis unit. Measurements were made of gaseous components, such as CO, CH4, acetylene, ethylene, and some aromatics, benzene, toluene, and ethylbenzene. The Temet FTIR gives a 2 ppm resolution with an accuracy of 2%. The instrument was calibrated by the manufacturers for the species of interest using reference gas concentrations, but the complexity of the mixtures would suggest greater errors than those given above. 2.2. Materials. The structure and boiling point of the compounds investigated in this study are listed in Table 1. The structure representing biodiesel is oleic acid methyl ester, but biodiesel also contains quantities of stearic, palmitic, and linoleic acid methyl esters.

3. RESULTS AND DISCUSSION 3.1. Capabilities of the System. The pyrolysis products of solid biomass fuels using a filament pyrolyzer, such as a CDS 5200, are dominated by primary pyrolysis products, as illustrated in Figure 2 for cellulose. Panels a and b of Figure 2 show the chromatograms obtained at 600 °C for the filament and the flow cell, respectively. At this temperature, the distribution of products is relatively similar and consists mainly of methoxyphenols, levoglucosan, and cyclopentanones. Panels c and d of Figure 2 show the chromatograms obtained from pyrolysis at 900 °C using a filament and flow cell pyrolyzer, respectively. At higher temperatures, more PAHs are formed using the filament pyrolyzer as expected but there is still a significant amount of primary

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Table 1. Compounds Studied

pyrolysis products present. This is due to a combination of the shorter residence time and faster quenching time during filament pyrolysis. In the flow cell experiments, the product profile is simpler and dominated by secondary hydrocarbon products. This is largely a result of more complete pyrolysis because of a longer residence time. Fewer oxygenated species are observed as the primary pyrolysis products are allowed to continue reacting. This result also demonstrates that the cellulose fraction of the biomass also contributes to PAH formation because the major products formed, include benzene, toluene, indene, and naphthalene. This result is difficult to observe using a filament pyrolyzer even at long residence times and slower ramp rates, although these species can be formed during the combustion of biomass when mixing is incomplete.12 It is difficult to investigate thermal decomposition of volatile liquids using the filament pyrolyzer, and there is significant breakthrough during analysis. The microflow cell allows for the injection of liquid model compounds directly into the cell using a syringe. The filament pyrolyzer is useful for investigating primary pyrolysis products; however, the residence times are not long enough for equilibrium to be reached. The flow cell has more flexibility and allows for variable residence time, allowing the secondary, thermodynamically favored products to be observed. This is particularly useful when investigating emission behavior from solid and liquid fuels. 3.2. Effect of the Temperature Variation. Variation of the pyrolysis temperature between 600 and 900 °C allows for investigation of the thresholds at which aromatics and PAHs begin to form in significant amounts. A number of different fuels and model compounds have been analyzed using this system. These include alkanes as well as oxygenated fuels, such as biodiesel and biomass pyrolysis intermediates. n-Hexadecane is in the mid-distillation range of diesel and, hence, is representative of the n-alkanes that comprise a large fraction of diesel fuel. 3.2.1. Effect of the Temperature on Hexadecane Pyrolysis. Figure 3 demonstrates the thermal decomposition and recombination of n-hexadecane during pyrolysis at four temperatures: 2947


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Figure 2. Comparison of pyrolysis fingerprints from pyrolysis of cellulose using (a) a filament pyrolyzer at 600 °C, (b) a flow cell pyrolyzer at 600 °C, (c) a filament pyrolyzer at 900 °C, and (d) a flow cell pyrolyzer at 900 °C.

600, 700, 800, and 900 °C. At 600 °C, the main peak is unreacted hexadecane. There are small traces of smaller alkenes and alkanes representing cracked fragments. Aromatic species were not detected at this temperature. At 700 °C, the reaction is more extensive and a range of alkenes and alkanes is found in a size range of 716 carbon atoms. These are present as doublets of peaks, and therefore, the dominant peak is labeled. There are two large peaks at the earliest retention times, corresponding to alkyl-cyclopropane and benzene. Naphthalene, phenanthrene, fluoranthene, and pyrene are all detectable in small amounts.

At a pyrolysis temperature of 800 °C, the unreacted hexadecane is still the most abundant species but the benzene peak is almost as large. The concentrations of cracked fragments (alkenes and alkanes) are much lower than was seen at 700 °C. There are now a wide range of small aromatic molecules, such as toluene, styrene, and naphthalene, present. Phenanthrene, fluoranthene, and pyrene are still only present in trace amounts. The chromatogram obtained at 900 °C shows that there is no longer any unreacted n-hexadecane. All of the largest peaks of the captured species can be attributed to either cyclic, aromatic, or polycyclic aromatic species. The most abundant species are benzene, toluene, 2948


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Figure 3. Chromatograms obtained using hexadecane at different pyrolysis temperatures. Peaks: a, octene; b, nonene; c, decene; d, undecene; e, dodecene; f, tridecene; g, tetradecene; h, pentadecene; i, n-hexadecane; 1, 1,3-cyclopentadiene; 2, benzene; 3, toluene; 4, m- þ p-xylene; 5, styrene; 6, indene; 7, naphthalene; 8, 2-methylnaphthalene; 9, 1-methylnaphthalene; 10, biphenyl; 11, acenaphthylene; 12, phenanthrene; 13, fluoranthene; and 14, pyrene.

styrene, and naphthalene. The fluoranthene and pyrene peaks are increased significantly compared to the 800 °C condition. 3.2.2. Effect of the Temperature on the Pyrolysis of Biomass Compounds. Figure 4 demonstrates the thermal decomposition and recombination of guaiacol during pyrolysis at four temperatures: 600, 700, 800, and 900 °C. Guaiacol is a degradation product of lignocellulosic biomass derived from the cellulose portion. At 600 °C, the main peak is unreacted guaiacol plus some additional degradation products. As the temperature increases, the concentrations of benzene, toluene, and styrene

increase. At 900 °C, the chromatogram does not contain any primary pyrolysis products but consists predominantly of aromatics, including two-, three-, and four-ring PAHs (the four-ring species are not shown in the chromatogram so as not to compress the chromatogram). The profile is similar to that from the n-hexadecane pyrolysis at 900 °C, and it was found that many of the fuels and model compounds studied showed this similar profile at the highest temperature. 3.3. Effect of the Residence Time Variation. A major advantage of linking the flow cell to the adsorbent trap is that 2949


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Figure 4. Influence of the temperature on the pyrolysis of guaiacol: 1, 1,3- cyclopentadiene; 2, benzene; 3, toluene; 4, m- þ p-xylene; 5, styrene; 6, indene; 7, naphthalene; 8, 2-methylnaphthalene; 9, 1-methylnaphthalene, 10, biphenyl; 11, acenaphthylene; and 12, phenanthrene.

the gas flow and, hence, residence time can be varied through the flow cell. This also allows reactant gases to be used without damaging the chromatographic column. The influence of the flow rate and residence time has been investigated for various compounds. The influence of changing the flow rate for the reaction at 900 °C is shown in Figure 5, and for furfural, the results recast as a function of the residence time are shown in Figure 6. For the furfural example, breakthrough is seen at the lowest residence time, and an increased residence time results in the reaction of furfural to benzene, toluene, styrene, and naphthalene as major products. Minimal breakthrough is observed at 50 mL/min.

Eugenol is a guaiacol-type compound, derived from lignin, whereas furfuryl alcohol is derived from the pentose sugars in cellulose. While both species exhibit similar profiles and trends for increased reaction times, there are some differences. Eugenol and guaiacol have higher intensity peaks, which may in part be accounted for by their lower oxygen content, 19 and 25.8%, respectively, compared to 32.7 and 33.3% for furfuryl alcohol and furfural and also because furans decompose to more C1C3 species, which cannot be detected by this system. Something that is common to all of the chromatograms is that benzene is always the dominant peak; however, for furan species, it is considerably 2950


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Figure 5. Influence of the flow rate on the formation of benzene, toluene, styrene, and naphthalene for (a) furfural, (b) furfuryl alcohol, (c) eugenol, and (d) guaiacol.

Figure 6. Effect of the residence time on pyrolysis products for furfural.

higher at all flow rates, whereas at shorter reaction times, it is almost equal to toluene and styrene for guaiacol chromatograms. At the highest flow rate, benzene, toluene, styrene, benzofuran, indene, and naphthalene are the main peaks for all chromatograms; also present to a lesser degree for all are xylene, methylnaphthalene, biphenyl, acenaphthylene, and fluorene, with small amounts of phenanthrene, anthracene, 2-phenylnaphthalene, and peaks corresponding to molecular weight 202 Da, fluoranthene, pyrene, and acephenanthrylene. Guaiacols display a more complex chromatogram of both cracking and rearrangement products, with additional species, such as substituted

benzenes, like (1-methylethenyl)benzene and 1-ethenyl-2methylbenzene. Simplification coupled with a decrease in relative intensity of the guaiacol chromatogram occurs by decreasing the flow rate to 10 mL/min (increasing residence time), resulting in a decrease in the relative amount of minor peaks because of further decomposition of substituted benzenes. This trend is less pronounced for furan species, although the relative intensity does decrease slightly; a reduced flow rate does not considerably change the chromatographic profile. At 10 mL/min, guiacols have lower intensity chromatograms and only the major peaks for benzene, toluene, and naphthalene, with trace amounts of xylene and styrene, are seen (and a small amount of unreacted compound for eugenol). In addition to these products, the furan species also have notable peaks for styrene, indene, benzofuran, methylnaphthalene, and biphenyl; this may be because the furan compounds require a longer reaction time for the formation of benzene and its homologues, because benzene is not present in the parent compound. Consequently, a longer reaction time may be required for the destruction of these other species. The reduced magnitude of species in the chromatograms with shorter reaction times could conceivably be due to increases in both C1C3 gases as a result of aliphatic benzenes losing substituents and also owing to PAH formation in species with more than four rings that are too large for detection within this system. Enlargments are presented in Figure 7 of the chromatogram of furfuryl alcohol, which illustrates the change between 3 and 5.5 min of C4C5 at varying flow rates. Furan and methylfuran 2951


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Figure 7. Magnification of furfuryl alcohol chromatogram between 3.0 and 5.5 min for different flow rates: 1, 1,3-butadiene; 2, 1,3-pentadiene; 3, furan; 4, CPD; and 5, methylfuran.

(a derivative of the parent compound) are initially present for furfuryl alcohol pyrolysis at shorter reaction times (100 and 50 mL/min). So too is CPD, which has a promenient peak, and there is a small peak for 1,3-butadiene (which is a slightly more distinct peak in the chromatogram at 20 mL/min). As the reaction time increases (flow rate decreases), these species decrease and butadiene appears. The longer residence times possible with this system allow further pyrolysis of fragments, which leads to increased PAH formation. PAHs are produced from both cellulose and lignin model compounds, although they are observed to increase with an increasing phenolic content. Pyrolysis of lignin and cellulose model compounds both produce indene and naphthalene as major products, and it is thought that CPD is an important intermediate for their formation. Figure 8 shows the relative distribution of species from pyrolysis of biomass model compounds and biodiesel at 900 °C, at a flow rate of 50 mL/min. It is clear that many of the species formed at 900 °C are cyclic or aromatic, regardless of the original compound structure, and this includes a range of materials, such as n-hexadecane, biodiesel, and the biomass decomposition intermediate products, furfuryl alcohol and guaiacol. Additional information has been obtained using FTIR analysis of the pyrolysis products from n-hexane, eugenol, and furfural, and the composition of the exit gases from a flow reactor are given in Table 2. Because equal volumes of reactant were injected, they have been normalized to peak values (ppm) per mole of reactant. The results are also shown for the same reaction time, but because the reactants have different thermal stabilities, it is not possible to directly compare the yields of products from each reactant. The results show general trends, although they are not as specific and accurate as those obtained using GCMS. That is, the major species are low-molecular-weight alkanes and unsaturated products, and there is a range of single-ring compounds and naphthalenes present for all of the reactants. n-Hexane produces

significant amounts of methane, ethylene, benzene, and trimethylbenzene, as well as naphthalene. The ratio of benzene/ naphthalene is 9.8:1 for hexane, 8.7:1 for eugenol, and 8.1:1 for furfural, suggesting a common mechanism for the formation of this two-ring compound. No such commonality holds for acetylene, suggesting that it is not involved in benzene formation at this temperature. In the case of the oxygenated species, a common pyrolysis product is CO, and CO2 is not present. 3.4. Relevance to PAH Formation during Pyrolysis and Combustion. A variety of mechanisms have been suggested as contributing to the formation of PAHs, which are soot precursors during the pyrolysis and combustion of hydrocarbon and oxygen-containing fuels, such as coal,12 biomass,11 and liquid-fuel droplets.13 For example, in systems ranging from laboratory burners to a large power station combustor,29 depending upon the fuel and the combustion temperature, the following radical reaction types have been proposed as contributing either sequentially or in parallel: (1) hydrogen abstraction followed by carbon addition through the reaction with C2 and C4 radicals,9,10,18 (2) reactions involving resonance-stabilized intermediates, especially benzyl, CPDyl, and propargyl,26,28,3034 and (3) arylaryl coupling of PAH radicals formed by hydrogen abstraction.32 These reaction types can also be invoked as occurring during pyrolysis of a range of fuels. At high temperatures, hydrocarbon fragments, such as C2 or C4, are significant. Bittner and Howard18 have extensively explored the role of C2 and C4 in the formation of PAHs in hydrocarbon flames, but at lower temperatures there is evidence for the involvement of resonance-stabilized free radicals, especially CPDyl. In the case of both engine biofuels and solid biomass, CPDyl may be produced from phenoxy radicals, which arise from phenols that, in turn, are products of biofuel or biomass decomposition.11,12 Another origin of CPDyl through the reaction of acetylene with propargyl has been proposed.35 CPDyl can combine to produce a range of PAHs, including indene and naphthalene. Reactions of CPDyl with naphthalene and indene can then lead to the growth of PAHs with three and four rings. 2952


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Figure 8. Flow cell pyrolysisGCMS chromatograms of (a) furfuryl alcohol, (b) guaiacol, and (c) biodiesel at 900 °C: 1, 1,3-cyclopentadiene; 2, benzene; 3, toluene; 4, m- þ p-xylene; 5, styrene; 6, indene; 7, naphthalene; 8, 2-methylnaphthalene; 9, 1-methylnaphthalene; 10, biphenyl; 11, acenaphthylene; and 12, phenanthrene.

CPD, naphthalene, and indene have been identified in the pyrolysis products of catechol27 and eugenol,11 and Mulholland et al. have proposed25,26,28 the formation from CPDyl of PAH products that could not be represented by either acetylene or arylaryl addition pathways. Mechanisms 13 above, along with contributions from phenyl radical addition, have been incorporated by Norinaga et al.17 into a computational chemical kinetic model, which accounts for the identity and relative concentrations of the products of pyrolysis of unsaturated light hydrocarbons in the temperature range of 8001100 °C. An alternative suite of reactions with the inclusion of methyl, phenyl, and benzyl but without CPDyl has been invoked by Shukla and Koshi16 to explain qualitatively the products of the pyrolysis of toluene. Other radicals that have often been implicated in the formation of aromatics, including PAHs and soot,3032 include highly stable35 propargyl, which has been proposed33 to react with benzyl; the latter radical is a

product of toluene pyrolysis and a major intermediate in PAH formation.36 Aromatics may also grow by reactions involving the phenyl radical in a scheme incorporating benzyl and methyl.16 These reaction schemes have in common that some or all of the reaction types may be involved to different extents in the pyrosynthesis of an individual product PAH. Thus, coronene is thought17 to arise by HACA reactions from perylene, itself formed by arylaryl coupling of 1-naphthyl with naphthalene, with the latter being formed from CPDyl. The reaction of phenyl with benzene can give biphenyl and then phenanthrene via HACA.16,33 The versatile flow-cell pyrolysisGCMS (with complementary pyrolysisFTIR) system described here allows for the distribution of products of pyrolysis of liquid and solid fuels to be determined for temperatures between 600 and 900 °C at controlled residence times and in the presence of either an inert or a reactive gas. In fact, the same PAHs with 24 rings and 2953


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Table 2. Pyrolysis Products Obtained by the FTIR Experiments for n-Hexane, Eugenol, and Furfural at 900 °C n-hexane relative eugenol relative furfural relative species methane ethane

579 260.5

acetylene

231.6

ethylene

1434.2

1,3-butadiene benzene 1,2,4-trimethylbenzene toluene ethylbenzene naphthalene

a

concentrationa

concentrationa concentrationa 5.9 5.6

10.2 6.8

5.6

152.5

20

11.9

192.1

5.0

5.0

28.1

7.8

8.1

147.4

4.1

2.7

2.9

9.1

5.0

28.9 2.9

3.1 0.9

1.5 1.0

87.50

CO

trace

CO2

∼0

∼0

’ AUTHOR INFORMATION

formaldehyde

∼0

4.1

22.5

Corresponding Author

acrolein

trace

2.5

16.9

*E-mail: [email protected].

acetic acid

trace

0.9

5.9

∼0

474.5

demonstrated because of the ability to vary residence time. A longer residence time ensures that primary pyrolysis products can continue reacting, so that more complete pyrolysis is observed. The technique is more suitable than the filament pyrolyzer for volatile liquids. This technique has been used to illustrate the potential to investigate biofuel decomposition, PAH growth mechanisms, and soot formation from different hydrocarbon and oxygenated fuels, where direct sampling of the products is required. Results indicate that, at the highest temperature measured (900 °C), the pyrolysis products contain aromatic species, which tend to be similar for the range of different sample types investigated. Evidence is obtained indicating that resonance-stabilized intermediates, particularly CPDyl, are important in PAH growth mechanisms for a wide range of fuel types, including oxygenated fuels.

Normalized to equimolar samples injected (see the text).

monocyclic (especially alkylbenzene) products were identified by pyrolysisGCMS from a number of oxygenated fuels (biodiesel, the cellulose model compounds, furfural and furfuryl alcohol, and the lignin models, eugenol and guaiacol), namely, indene, naphthalene, methylnaphthalene, biphenyl, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzene, toluene, and the C2 benzenes (xylene, ethylbenzene, and styrene). The upper mass limit is imposed by the limitations of the operating conditions of the GC column. PyrolysisFTIR also showed the production of formaldehyde, acetic acid, and acrolein. The pyrolysis of hexadecane produced the same list of hydrocarbons, except for the absence of fluorene and anthracene and the presence of C3 alkylbenzenes. The remarkable similarity of these products from different compound types, taken with the above mechanistic considerations, points to the generation of a common pool of reactive precursors, the identities of which are independent of the substrate; in confirmation, the parent compounds of the benzyl, CPDyl, indenyl, phenyl, and propargyl radicals, along with acetylene and butadiene, were always present. This analysis corroborates engine studies that have reported significant PAH emissions when using biodiesel and other alternative fuels.37 PAH is also formed in the early stages of combustion of dimethyl ether in a counterflow flame.38 There is a similarity in using conventional fuels and biofuels for both PAH and soot formation.37,39 The formation of NO is slightly enhanced by the use of biofuel.37,40 This has been attributed to differences in the ignition process but also because of the increase in unsaturated species,40 which can influence the temperature but also prompt NO formation.

4. CONCLUSION Evidence of PAH growth mechanisms can be provided by analytical pyrolysis experiments using high-temperature flow cells. An analytical micropyrolysis flow cell method has been developed, enabling reactive pyrolysis of solid and liquid samples. Benefits of the system over a filament pyrolyzer have been

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