Mechanistic Aspects of Soot Formation from the Combustion of Pine

Energy & Fuels 2008, 22, 3771–3778

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Mechanistic Aspects of Soot Formation from the Combustion of Pine Wood E. M. Fitzpatrick,*,† J. M. Jones,† M. Pourkashanian,† A. B. Ross,† A. Williams,‡ and K. D. Bartle† Energy and Resources Research Institute, UniVersity of Leeds, Leeds, LS2 9JT, United Kingdom, and Centre for Computational Fluid Dynamics, Houldsworth Building, UniVersity of Leeds, LS2 9JT, United Kingdom ReceiVed June 13, 2008. ReVised Manuscript ReceiVed September 9, 2008

Carbonaceous soot has been generated from pine in a range of appliances to simulate different combustion conditions. The fuel as well as biomass cell wall components have been studied by pyrolysis-GC-MS and pyrolysis-GC-TCD. In addition, the soots have been probed using both pyrolysis-GC-MS and direct inlet mass spectrometry (DI-MS). The material collected from the pine combustion is smoke, and the major component is a carbonaceous soot. The soots contain both organic carbon (adsorbed species) and black (solid) soot, and the organic carbon consists of primary pyrolysis products from the cell wall components, as well as decomposition products, PAH and oxidized PAH. The black carbon contains oxygen functionality (of the order of 5-10 wt % O), and there are indications that this is incorporated during soot growth, although surface oxidation on reactive sites could also be important. The decomposition products suggest an important additional PAH route is via cyclopentadiene, which is derived after cracking of lignin monomer fragments. Kinetic modeling also highlights the lignin monomers as important contributions to the soot production pathways. A model is proposed which, in addition to the hydrogen abstraction carbon addition (HACA) mechanism, incorporates the cyclopentadiene and the O-PAH addition routes to soot.

Introduction The contribution of particulate from the combustion of wood is considered to be important because globally it is a major domestic fuel, and a considerable number of deaths are attributed to the effects of wood smoke.1-3 Smoke can also be formed from accidental and forest fires where the toxic products form a major hazard.4 Consequently, the properties of smoke from biomass combustion have been studied over many decades. It has been established that combustion generated particulates have an important impact on climate and rainfall.5,6 In atmospheric modeling, it is recognized that smoke consists of soot, volatiles, and ash, and the soot itself consists of two components, black soot and organic soot. Recently it has been recognized that the black soot consists of two components, elemental soot and condensed organic compounds.7-9 For aerosols produced from biomass burning, the organic carbon * To whom correspondence should be addressed. E-mail: ces4emf@ leeds.ac.uk. † Energy Resources Research Institute. ‡ Centre for CFD. (1) Ramanathan, V.; Crutzen, P. J.; Kiel, J. T.; Rosenfeld, D. Science 2001, 294, 2119–2124. (2) Graf, H. F. Science 2004, 303, 1309–1311. (3) Nacher, L. P.; Brauer, M.; Lipsett, M.; Zelikoff, J. T.; Simpson, C. D.; Koenig, J. Q.; Smith, K. R. Inhal. Toxicol. 2007, 19, 67–106. (4) Fitzpatrick, E. M.; Ross, A. B.; Bates, J.; Andrews, G. E.; Jones, J. M.; Phylaktou, H.; Pourkashanian, M.; Williams, A. Process Saf. EnViron. IChemE. 2007, 85 (b5), 430–440. (5) Crutzen, P. J.; Andreae, M. O. Science 1990, 250, 1669–1678. (6) Andreae, M. O.; Rosenfeld, D.; Artaxo, P.; Costa, A. A.; Frank, G. P.; Longo, K. M.; Silva-Dias, M. A. F. Science 2004, 303, 1337–1342. (7) Penner, J. E.; Chuang, C. C.; Grant, K. Clim. Dynam. 1998, 14, 839–851. (8) Grant, K.; Chuang, C. C.; Grossman, A. S.; Penner, J. E. Atmos. EnViron. 1999, 33, 2603–2620.

content is high, and this is thought to have a huge impact on the warming potential of the elemental carbon, even to the extent of neutralizing the warming effect.7,8 For primary emissions from fossil fuel, the organic carbon/elemental carbon ratio is lower, and thus the soot has an overall warming effect.7,9,10 Thus, particulate matter from biomass combustion has very different properties and adsorbed species compared to particulate from hydrocarbon combustion. Biomass has greater inherent oxygen functionalities, and the carbonaceous aerosols arising from combustion act as a carrier for adsorbed chemical species. It is proposed that biomass soot particles contain oxygen as part of the particle structure, and surface oxygenates make it hydrophilic, thus aiding absorption of many species.11 The aim of the paper is to provide and review evidence for the additional mechanistic routes. Oxygen-containing compounds appear to play a significant role in the production of wood soot.11,12 Results suggest that there are two routes by which solid soot can be synthesized:13,14 first through conventional hydrocarbon mechanisms such as the HACA (hydrogen (9) Cooke, W. F.; Liousse, C.; Cachier, H.; Feichter, J. J. Geophys. Res. 1999, 104, 22137–22162. (10) Schaap, M.; Denier van der Gon, H. A. C. Atmos. EnViron. 2007, 41, 5908–5920. (11) Jones, J. M.; Ross, A. B.; Williams, A. J. Energy Inst. 2005, 78, 199–200. (12) Ross, A. B.; Junyapoon, S.; Jones, J. M.; Williams, A.; Bartle, K. D. J. Anal. Appl. Pyrol. 2005, 74, 494–501. (13) Fitzpatrick, E. M.; Ross, A. B.; Jones, J. M.; Williams, A. Formation of Soot and Oxygenated Species from Wood Combustion, European Combust. Meeting 2007, Chania, Greece, April, 2007. (14) Fitzpatrick, E. M.; Ross, A. B.; Jones, J. M.; Williams, A. Smoke Produced from the Combustion of Biomass, 15th Biomass Conference and Exhibition, Berlin, May, 2007.

10.1021/ef800456k CCC: $40.75  2008 American Chemical Society Published on Web 10/28/2008

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Figure 1. (a) Thermal desorption of soot from high-temperature pine wood combustion and (b) py-GC-MS of pine wood at 1000 °C.

abstraction carbon addition)15,16 method and second through a route involving the reaction and transformation of biomass pyrolysis fragments. This can be observed from products adsorbed onto the soot particles thought to be intermediates between smaller oxygenated fragments and more highly developed soot. Evidence is presented of an alternative route for initial PAH growth in biomass combustion via a cyclopentadiene intermediate. Experimental Methods The composition of the pine on a dry basis is: C, 47.5 wt %; H, 6.1 wt %; N, 0.2 wt %; volatile matter, 86.2 wt %; and ash, 0.4 wt %. The pine consists of 50% cellulose, 20% lignin, and 6% moisture, and the remainder is hemicellulose and ash. The formation of smoke from wood combustion in a fire chamber, as described previously,4 consists of a chamber (dimensions 1.4 m × 0.92 m × 1.22 m) from which the combustion products can be sampled by FTIR and smoke probe sampling. Temperatures in the sampling region ranged from 400 to 750 °C (15) Frenklach, M.; Wang, H. Soot Formation in Combustion: Mechanisms and Models; Bockhorn, H., Eds.; Springer-Verlag: Berlin, 1994; pp 162-192. (16) Frenklach, M.www.me.berkeley.edu/soot/.

from the edges of the flame. Samples of soot from eugenol and benzene were obtained using a small diffusion flame burner previously described.11,12 A stainless steel sampling probe was used, and soot samples were collected at 40 °C onto quartz binderless filter papers (SKC type R-100, 1.2 µm, 37 mm diameter). This technique replicates the conditions when products from a biomass flame or flue gases from a biomass furnace enter the atmosphere, and the temperature that determines the EC and OC contents of the soot particles. It is almost identical to that used by Santamaria et al.17 and similar to that used in a number of other studies in which wood smoke is sampled from a combustor, for example ref 18. The same binderless filter papers were also used for the collection of combustion soot from eugenol and benzene flames. Samples were stored in a refrigerator in sealed containers. Pyrolysis of pine (particle size 75-90 µm) was studied using a CDS 5000 pyrolyser connected to a Shimadzu 2010 gas chromatograph with mass spectrometric detection (py-GC-MS). The products were separated on an Rtx 1701 60 m capillary column, 0.25 mm i.d., 0.25 µm film thickness, using a temperature program of 40 °C, hold time of 2 min, ramped to 250 °C, hold time of 30 min, and a column head pressure at 40 °C of 30 psi. Gas yields (17) Santamaria, A.; Mondragon, F.; Molina, A.; Marsh, N. D.; Eddings, E. G.; Sarofim, A. F. Combust. Flame 2006, 146, 52–62. (18) Klippell, N.; Nussbaume, T. Paper W 1612, 15th European Biomass Conference and Exhibition, Berlin, 7-11 May, 2007.

Mechanistic Aspects of Soot Formation of Pine Wood

Energy & Fuels, Vol. 22, No. 6, 2008 3773

Figure 2. Gas yield as a function of pyrolysis temperature for ([) pine, (4) lignin, (0) cellulose, (×) xylan (from oat spelt): (a) hydrogen, (b) carbon monoxide, (c) methane, (d) carbon dioxide.

were measured using a CDS 1000 pyrolyser connected to a PyeUnicam Series 204 GC with a thermal conductivity detector, equipped with a 2 m column containing molecular sieves for separation of CO, CH4, and H2, and silica gel for determination of CO2. Studies of the pyrolysis and oxidative pyrolysis of intermediate products (eugenol, etc.) were also made using a pyrex glass flow cell, connected to the CDS 5000 interface unit to focus products into the GC-MS, operated at 700 °C with a 0.8 s residence time and reactant and oxygen concentrations estimated at 1000 ppm in a helium carrier gas. The combustion soot samples were analyzed directly by pyGC-MS. Samples of 2 mg were removed from the filters and placed between quartz wool in a thin-walled pyrolysis tube and heated at two temperatures, first for thermal desorption of the adsorbed organic carbon at 400 °C and second for pyrolysis of the soot at 1000 °C. For each temperature, a ramp rate of 20 °C ms-1 with a hold time of 20 s is employed. Fire chamber experiments were repeated several times and under different air-flow conditions,4 but soot samples show similar py-GC-MS patterns for all cases irrespective of fire conditions, exhibiting only slightly different peak intensities rather than different species. For all GC-MS studies, the chromatograms were assigned on the basis of the NIST 05 Mass Spectral Library Database, from previous literature19,20 and by known retention times. Direct inlet mass spectrometry (DI-MS) was performed using a Shimadzu 2010 GC-MS from 50 to 600 Da, and 1 mg of sample was heated from 25 to 500 °C at a ramp rate of 40 °C min-1.

Results and Discussion Thermal Desorption and Pyrolysis. The thermal desorption GC-MS chromatogram (at 400 °C) of the combustion soot is shown in Figure 1a. Both PAH and oxygenated PAH are present. The soot contains absorbed material and when heated loses 27 wt % at 400 °C and a further 6% at 1000 °C, 33% in total. Additional information was gained from studies of species produced by the thermal decomposition of pine at various (19) Simoneit, B. R. T. Appl. Geochem. 2002, 17, 129–162. (20) Lemieux, P.; Lutes, C. C.; Santoianni, D. A. Prog. Energy Combust. Sci. 2004, 30, 1–32.

temperatures from 600 to 1200 °C in the pyroprobe unit. Figure 1b is a typical result obtained for the medium tars, although the types of species obtained change as the temperature is reduced, as reported previously.11 Figure 2 shows the gas yields from pine and its main cell wall components as a function of pyrolysis temperature. The total weight loss for pine, cellulose, and xylan increased with temperature (600-1200 °C) from 60 wt % to between 70 and 90 wt %. In contrast, lignin weight loss increased from 33 wt % at 600 °C to 64 wt % at 1200 °C. The contribution of the measured gases to the total weight loss was approximately 50%, and the remaining weight loss is attributed to light volatile organic compounds (VOC) and medium and heavy tars (as shown in Figure 1b for pine). Hydrocarbon gases and lighter VOC were detected directly during pine combustion4 using in situ FTIR spectrometry, calibrated for a number of species including acetylene and benzene. The evolutions of these species during combustion of a pine crib are shown in Figure 3 and give evidence for the role of acetylene in the soot-forming routes. Of the VOC and medium tars shown in Figure 1b, some of the dominant primary products are eugenol, isoeugenol, vanillin and guiacols (from lignin), and furfuryl alcohol, furfural, and levoglucosan from the hollocellulose. Some of these products, including furfuryl alcohol and eugenol, were studied in further pyrolysis and oxidative pyrolysis experiments, and results are given in Figure 4. Both model compounds produce PAH. Naphthalene and benzofuran were detected for furfuryl alcohol, while eugenol pyrolysis displays a more complex chromatogram of both cracking and rearrangement products, and also PAH (Figure 4b). In the presence of a small amount of oxygen, the abundance of all species decreases, but some new species are observed, including cyclopentadiene and oxidized cyclic and bicyclic products (Figure 4c). This is consistent with the general mechanism of biomass pyrolysis set out by Evans and Milne.21,22 In this model, the cellulose and lignin components can be considered separately, (21) Evans, R. J.; Milne, T. A. Energy Fuel 1987, 1, 123–137.


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Figure 3. Online FTIR concentrations of combustion products: (+) acetylene, (2), benzene, (9), ethylene, (b), 1,3-butadiene and (]) propene obtained from pine crib combustion with an air flow rate of 20 kg h-1 (adapted from ref 4).

and the cellulose decomposes readily at high temperatures to CO (and H2). GC-MS shows that the principal initial products include carbohydrate-derived material such as hydroxyacetaldehyde, acetic acid, hydroxypropanone, and levoglucosan and lignin-derived material such as furans, guiacols, syringols, and phenols. Pyrolytic char and water also accompany this. As the temperature increases beyond 500 °C, the guiacols, syringols, and furans begin to decrease. As the pyrolysis temperature increases further still (600-800 °C), the simple acids, aldehydes, ketones, and alcohols decrease, and more complex polycyclic oxygenates are formed, including benzofurans and benzaldehydes. The phenols are more stable and are still present at higher temperature. Above 900 °C, polycyclic aromatic compounds (PAC) become significant species (Figure 1a). The molecular weight profile of the higher PAC resulting from pine wood combustion was obtained by a temperatureprogrammed probe DI-MS of the soot and compared with soot from benzene combustion (Figure 5a and c). Although these samples were not collected under conditions favoring retention of lower-MW material, they are consistent with the HACA15 mechanism of PAH growth: both are dominated by a series of peaks separated by 24 or 50 Da and typical of the PAH found in soot from a wide range of fuels and thought to arise by sequential addition of C2 and C4 fragments23,24 to the growing molecule. Violi et al.25 have drawn attention to the dominance of a triad of PAH species with MW 252, 276, and 300 Da in the mass spectra of flame-generated soots from a variety of gaseous and liquid fuels, including benzene and ethene.23 The DI-MS spectra (Figure 5a) of pine wood soot also showed significant contributions from these species, but greater than (22) Evans, R. J.; Milne, T. A. Energy Fuel 1987, 1, 311–319. (23) Smedley, J. M.; Williams, A.; Bartle, K. D. Combust. Flame 1992, 91, 71–82. (24) Smedley, J. M.; Williams, A.; Bartle, K. D. Polycycl. Aromat. Comp. 1994, 4, 61–70. (25) Violi, A.; Sarofim, A. F.; Truong, T. N. Second Mediterranean Combustion Symposium, Sharm El-Sheik Egypt, Jan. 2002; pp 1161-1170.

the intensity of any of these was that of a peak corresponding to 202 Da. Thermal desorption GC-MS (Figure 1a) allowed attribution of this to fluoranthene and pyrene, along with a smaller contribution from acephenanthrylene. The cyclopentafused PAHs such as acephenanthrylene are commonly associated with the HACA mechanism and soot formation in hightemperature combustion. These results suggest a route to pine wood soot PAH different from that to PAH in soots from the combustion of gaseous and liquid hydrocarbons; the contribution of PAH containing five-membered rings (e.g., 202, 226, and 276, etc.) is consistent with a more important role for such structures in pine wood soot formation. The origin of pine wood soot precursors has been sought in the reactions of the terpene and lignin components, beginning with the reaction of small radicals with monocyclic and dicyclic aromatic hydrocarbons generated during the early stages of decomposition to yield large PAH.23 Two routes from biomass to single ring aromatics may be implicated: degradation reactions of terpenessthermal isomerization and dehydrogenation reactions of volatile minor wood constituents such as pinenes at temperatures up to 500 °Csand a more important and complex reaction sequence from the phenolic constituents of lignin. Here, eugenol is chosen as a model compound for the structural units of lignin, and it is demonstrated that on oxidative pyrolysis it generates mainly benzene, toluene, and the C2benzenes (Figure 4c). The major aromatic product from eugenol during pyrolysis at the same temperature (without oxygen) was naphthalene. Also identified were indene, methylnaphthalenes, acenaphthylene, fluorene, and phenanthrene, presumably because PAH formation is not now inhibited by reaction of radical intermediates with oxygen. Thermal desorption GC-MS of the eugenol soot showed the presence of rearrangement products and derived phenols. The DI-MS spectrum (Figure 5b) of eugenol soot confirms the presence of numerous oxygencontaining species and is also consistent with the presence of some PAH, with growth from naphthalene (MW 128 Da)



Figure 4. Pyrolysis of (a) furfuryl alcohol, (b) eugenol (E), and (c) oxidative pyrolysis of eugenol.

through phenanthrene (178) and the 252, 276, and 300 triad, with the 202 Da species again prominent. These results correspond closely to those of Wornat et al.26-28 who pyrolyzed catechol (o-dihydroxybenzene) as a representative of the lignin structure at temperatures between 500 and 1000 °C in the presence of varying amounts of oxygen. Below 800 °C, PAH products obtained under both pyrolysis and oxygen-rich conditions were two-ring compounds, especially indene and naphthalene, while above 800 °C in the absence of oxygen, much larger PAHs were generated in yields which decreased with increasing oxygen content. The results of these model-compound experiments provide an explanation for our previous observations employing a fixed(26) Wornat, M. J.; Ledesma, E. B.; Marsh, N. D. Fuel 2001, 80 (12), 1711–1726. (27) Thomas, S.; Wornat, M. J. Fuel 2008, 87, 768–781. (28) Thomas, S.; Ledesma, E. B.; Wornat, M. J. Fuel 2007, 86, 2581– 2595.

bed furnace.29 Using analytical methods designed to avoid losses of smaller volatile compounds, we found that the PAHs in both flue gas and particulate collected during the combustion of pine wood, both as sawdust briquettes and lump wood, are dominated by naphthalene and methylnaphthalenes. Single-ring aromatics (flue gas) and higher PAH were also generated, the latter in amounts decreasing monotonically with MW which were consistent with an origin in C2/C4 addition from smaller aromatic hydrocarbons, particularly naphthalene. Dufour et al.30 and Fardell et al.31 also reported indene and naphthalene as the major PAH products from the pyrolysis and low-ventilation combustion of wood. Clearly, the origin for PAH during biomass combustion lies in the generation of one- and especially two-ring aromatics by the thermolysis of phenols derived from lignin as shown in (29) Ross, A. B.; Jones, J. M.; Chaiklangmuang, S.; Pourkashanian, M.; Williams, A.; Kubica, K.; Andersson, J. T.; Kerst, M.; Danihelka, P.; Bartle, K. D. Fuel 2002, 81, 571–582.


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Figure 5. Average mass spectrum obtained via direct inlet mass spectrometry of soot from (a) pine wood combustion, (b) eugenol diffusion flames, and (c) benzene diffusion flames. Scheme 1. Outline Biomass Combustion Mechanism, Showing the Main Routes to Soot and Char

Schemes 1 and 2. The mechanism of thermal decomposition of phenol and its methylated derivatives was established by Cypres and Bettens,32-34 who employed 14C- and 3H-labeled compounds and radiochromatography to prove that, rather than dehydroxylation and demethylation, the primary reaction is elimination of carbon monoxide to yield cyclopentadiene (CPD) and its methyl derivatives. Pyrolysis of CPD then rapidly gives rise35-38 to benzene, toluene, indene, and naphthalene as major products; (30) Dufour, A.; Girods, P.; Masson, E.; Normand, S.; Rogaume, Y.; Zoulalian, A. J. Chromatogr. A 2007, 1164, 240–247. (31) Fardell, P. J.; Murrell, J. M.; Murrell, J. V. Fire Mater. 1986, 10, 21–28. (32) Cypres, R.; Bettens, B. Tetrahedron 1974, 30, 1253. (33) Cypres, R.; Bettens, B. Tetrahedron 1975, 31, 353. (34) Cypres, R.; Bettens, B. Tetrahedron 1975, 31, 359. (35) Spielmann, R.; Cramers, C. A. Chromatographia 1972, 5, 295– 300. (36) Mulholland, J. A.; Lu, M.; Kim, D. H. Proc. Combust. Inst. 2000, 28, 2593–259935. (37) Mulholland, M. L. Chemosphere 2004, 55, 605–61036.

methylnaphthalenes derive from methylcyclopentadienes. Since CPD reacts more rapidly than phenols, indeed at temperatures as low as 550 °C, only small concentrations of this key intermediate are expected during the pyrolysis of phenols (see, for example, the small peak originating from CPD in the GC-MS chromatogram from eugenol pyrolysis products) (Figure 4). Catechol pyrolysis yields28 cyclopentadiene among light hydrocarbon products. In summary, there is evidence consistent with PAH growth from both HACA and via radicals other than C2/C4, namely, an aromatic route40,41 The detection of both C2/C4 gas phase species (Figure 3) together with the mass spectrometry data in Figure 5 is consistent with a major HACA contributor to PAH growth.42 An important route to naphthalene is via CPD from phenolic pyrolysis products, and this can react further via either HACA or other radical addition reactions. Oxygen in Soot. The amounts of soot produced from a pine crib fire4 were 29 mg m-3 for a 1 kg wood sample burning with an airflow of 10 kg h-1 and decreased to 10 mg m-3 with a higher 40 kg h-1 airflow rate. The results presented in Figures (38) Melius, C. F.; Colvin, M. E.; Marinov, N. M.; Pitz, W. J.; Senkan, S. M. Proc. Combust. Inst. 1996, 26, 685–692. (39) Wang, D.; Violi, A.; Kim, D. H.; Mullholland, J. A. J. Phys. Chem. A 2006, 110, 4719–4725. (40) D’Anna, A.; Violi, A. Energy Fuel 2005, 19, 79–86. (41) Echavarria, C. A.; Sarofim, A. F.; Lighty, J. S.; D’Anna, A. Proc. Comb. Inst. 2008, 32, to be published. (42) Apicella, B.; Millan, M.; Herod, A. A.; Carpentieri, A.; Pucci, P.; Ciajolo, A. Rapid Commun. Mass Spectrom. 2006, 20, 1104–1108.



Scheme 2. Reaction Pathways in the Decomposition of Lignin Species and Interaction with the Soot-Forming Speciesa

a

References for each reaction step are given above the arrows; [/] is proposed by this work.

1a and 5 are consistent with the presence of primary, secondary, and tertiary oxygenated products adsorbed on the soot. Figure 4 illustrates that the tertiary PAH products can arise from primary decomposition products of both cellulose and lignin. Oxygenated PAHs are also apparent in the pyrolysis of pine and eugenol and in the oxidative pyrolysis of eugenol. Of interest is whether these PAHs can be incorporated into the soot growth mechanism and so result in oxygenated soot. Electron energy loss spectroscopy (EELS) shows an oxygen content of 3.6-6.8%. The actual organic oxygen contents are higher than this since some contamination from the quartz fiber filters was present. If the analysis is corrected for this, then the measured oxygen content increases. For example, soot from a wood stove (as collected, unwashed) had a composition of C 94.3, H 0.96, and O 4.74 wt %. SEM images of biomass soot15,43 have similar topography to hydrocarbon soots.43 TEM images of soot from the combustion chamber were compared to decane and eugenol soot. In keeping with results found by Vander Wal and Muellery,45 preliminary work showed that the amount of disorder within the soot nanostructure increased as the oxygen content of the parent fuel increased. The decane soot has discernible lamella which form onion-like concentric rings; the eugenol has more disordered regions of lamella; and the wood soot is largely amorphous with only a few pockets of graphitic structure. An XPS comparison of pine soot and benzene soot demonstrates the existence of surface oxygen groups in both samples although more intense in the biomass soot, and clear differences in the oxygen functionalities were evident. The (43) Evans, M.; Vithandurage, I.; Williams, A. J. Inst. E. LIV 1981, 179–186. (44) Evans, M.; Williams, A. Fuel 1981, 60, 1047–1056. (45) Vander Wal, R. L.; Muellery, C. J. Energy Fuel. 2006, 20, 2364– 2369.

structural nature of particles has an implication on the hydrophilic nature of the soot particles and hence an impact on climate effects.1,2,5,6,11 Mechanism of Soot Formation. The major reaction routes in the flame were replicated by a PSR model.46 The reaction conditions of the smoke-forming region in the burning wood crib described in ref 4 were taken to be a temperature of 1500 K and a residence time of 0.1 s. Yii et al.47 report similar flame zone temperatures for a wood fire crib, and Klason Bai48 for a wood burning stove give similar temperatures. The residence time is based on the experimental gas flow rates. The basic steps of the proposed chemical mechanism are set out in Scheme 1, and it consists of three parts: (I) Hydrocarbon pyrolysis/oxidation followed by the HACA mechanism where species derived from the biomass components form acetylene and develop into PAH species products Ar1 to Ar4 (where Ar is an aromatic benzene ring).15,16 (II) Soot nucleation model: the soot precursors nucleate and grow according to the Frenklach model.16 In this model, nucleation takes place by the association of the Ar4 species. The initial soot particles grow to larger spherical particles. SEM measurements show that the particle sizes are approximately 30 nm and agglomerate or aggregate to form chains. For this part of the mechanism, there is no difference between soot from hydrocarbon fuels and from biomass. The model takes into account both the formation of soot and any oxidation of the soot particles produced. (46) Glarborg, P.; Kee, R. J.; Grcar, J. F.; Miller, J. A. PSR: A FORTRAN Program for Modeling Well-Stirred Reactors, Sandia Report SAND868209, Sandia National Laboratories: Albuquerque, NM 87185, 1991. (47) Yii, E. H.; Buchanan, A. H.; Fleischmann, C. M. Fire Safety J. 2006, 41, 62–75. (48) Klason, T.; Bai, X. S. Fuel 2007, 86, 1465–1474. (49) Mulholland, M. L. Chemosphere 2001, 42, 625–633.


(III) An additional route involving the reactions of biomass pyrolysis fragments. A representative species is eugenol which undergoes side-chain cracking, followed by conventional phenol decomposition reactions, and also decomposition and reaction via cyclopentadiene, as illustrated in Scheme 2. (IV) The combination of O-containing PAH groups. The HACA model only predicts an Ar4 PAH, which dimerizes to yield soot. The C/H and C/O ratios in actual soot are different and can be addressed in this way. Larger PAH species were invoked along the lines proposed by Violi et al.,25,50 and these include PAH-O species to explain the oxygen content of the soot. The first two routes were tested to examine the contributions from typical cellulose and lignin pyrolysis products. In the first case, cellulose is burned under diffusion flame conditions, that is, with a slightly rich (0.6) fuel/air ratio. It was found that the fractional soot volume was 1.3 × 10-9. Conversely, if the model lignin product, eugenol, is burned under the same conditions, the soot yield is 2.1 × 10-8. In the third case, pine wood (C40 H58 O23 N0.6), which consists of 80% cellulose and 20% lignin, is considered to be burned under the same conditions, and the soot yield is 5.0 × 10-9. This corresponds to a soot concentration of 10 mg m-3 which is of the same order as the measured values, 29 mg m-3, from pine combustion4 and although lower is reasonable given the assumptions. Clearly, lignin pyrolysis compounds of the type represented by eugenol favor soot production. The model used here does not describe the explicit soot-forming routes but only describes the tendency to form the HACA intermediates. This model cannot form oxygenated soot at present, but by considering the inclusion of O-PAH to the growing HACA PAH model can approximate the yield. From Figure 1, the relative importance of O-PAH and PAH can be estimated, and if both types of species are considered to contribute to soot growth, the O-content in the soot will be approximately 1-5 wt %. Although there are uncertainties still in the HACA mechanism,40,50,51 it can give a reasonable estimate of the soot yields from this type of flame. A molecular reaction scheme consistent (50) Violi, A.; Sarofim, A. F.; Voth, G. A. Combust. Sci. Technol. 2004, 176, 991–1005.

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with steps (a) and (b) in Scheme 1 is outlined in Scheme 2 showing the routes by which PAH can be derived from eugenol. It is clear from the modeling results that the amount of soot is dependent on the cellulose/lignin ratio. The soot precursors and the soot itself contain a small amount of oxygen, and C25H3O would be the smallest unit, a seven-ring structure linked to other similar structures, to form the soot particle (this would assume that the particle and the adsorbed species have the same composition) in the way proposed by Violi et al.50 Oxygen in soot has also been observed previously in soot from diesel engines.45 The cyclization of rearrangement products (Scheme 2) provides an alternative, so far unexplored, route to aromatic growth but one that would account at least in part for the oxygen content of pine wood soot. Conclusions A range of pine soots have been studied which contain both organic carbon and black carbon, and the nature of the organic carbon has been probed by py-GC-MS. It consists of primary pyrolysis products from the cell wall components, as well as decomposition products, PAH, and oxidized PAH. The black carbon (elemental) contains oxygen functionality (of the order of 5-10 wt % O), and there are indications that this is incorporated during soot growth, although surface oxidation on reactive sites could also be important. The decomposition products suggest an important PAH route is via cyclopentadiene, which is derived after cracking of lignin monomer fragments. Kinetic modeling also highlights the lignin monomers as important contributions to the soot production pathways. A model is proposed which, in addition to the HACA mechanism, incorporates the cyclopentadiene and the O-PAH addition routes to soot. Acknowledgment. The authors thank the EPSRC for support under grant number EP/C516974. E.M.F. thanks the Supergen Bioenergy Consortium for partial support and Professor Rik Bryson (LEMAS) for TEM and EELS measurements. EF800456K (51) Santamarıa, A.; Mondragon, F.; Quinonez, W.; Eddings, E. G.; Sarofim, A. F. Fuel 2007, 86, 1908–1917.

Mechanistic Aspects of Soot Formation from the Combustion of Pine

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