Soot Formation from the Combustion of Biomass Pyrolysis Products

School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, United ... Energy Technology and Innovation Initiative (ETII), University of Lee...
0 downloads 0 Views 4MB Size
Download PDF
Article pubs.acs.org/EF

Soot Formation from the Combustion of Biomass Pyrolysis Products and a Hydrocarbon Fuel, n‑Decane: An Aerosol Time Of Flight Mass Spectrometer (ATOFMS) Study J. M. Wilson,†,⊥ M. T. Baeza-Romero,‡ J. M. Jones,§ M. Pourkashanian,∇ A. Williams,*,∇ A. R. Lea-Langton,§ A. B. Ross,§ and K. D. Bartle§ †

School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, United Kingdom Escuela de Ingeniería Industrial de Toledo, Universidad de Castilla la Mancha, 45071, Toledo, Spain § Energy Research Institute, University of Leeds, Leeds, LS2 9JT, United Kingdom ∇ Energy Technology and Innovation Initiative (ETII), University of Leeds, Leeds, LS2 9JT, United Kingdom ‡

ABSTRACT: This paper is concerned with an aerosol time-of-flight mass spectrometer (ATOFMS) study of soot formation from the combustion of proxies of biomass (eugenol, furfural) and a hydrocarbon fuel (n-decane). The objective of this work was to gain insight into the soot growth mechanism in the combustion of biomass by studying the combustion of single components of wood (eugenol−lignin model and furfural−cellulose model), and by comparison with soot composition from combustion of a hydrocarbon fuel whose soot-forming mechanism is better known. Liquid fuels were burned using a wick burner, and the products in the aerosol phase were examined using an ATOFMS. The reaction process for n-decane combustion was examined using an opposed flame simulation with Chemkin-Pro modeling. A comparison of the model output with experimental results for n-decane give information on the soot growth mechanism. The same main routes for soot formation were operative both in biomass proxies and in n-decane. The principal differences in the mechanism observed for eugenol and furfural versus n-decane are described. Mass spectral analysis indicated that a channel involving the propargyl radical is more important in furfural combustion than for the rest of the fuels. Eugenol mass spectrometry (MS) indicates the presence of the important HACA (hydrogen abstraction acetylene addition) route, producing large polycyclic aromatic hydrocarbons (PAHs). Moreover, this study gives evidence that not only lignin components contribute to soot formation in biomass combustion, but furfural, which is a cellulosic component, can also contribute, and the soot formation routes involved are different.

■

radicals;13 (iv) aryl−aryl coupling reactions of PAH radicals to give aromers;14,15 and (v) addition-cyclizations involving phenyl (PAC) and methyl (MAC) at fusing PAH sites.16 Much less is known about the mechanism and properties of PAH and soot particles from biomass combustion. In previous work, including the use of an aerosol time of flight mass spectrometer (ATOFMS) without any dilution stage, we have shown17−21 how initial biomass soots contain oxygenated compounds that later generate PAH and alkylated and oxyPAH adsorbed on carbonaceous particles. This is related to the ratio of elemental carbon to total carbon (EC/TC), where TC encompasses all carbon-containing species and gives an indication of condensed organic species on soot particles. Based on offline measurements, which must be confirmed by online experiments, it has been proposed19 that cellulosic components of biomass decompose in a flame first into oxygenated compounds such as furfural (which then decomposes into CO and H2), while the lignin decomposes into guaiacol, syringol, and eugenol, the latter being a major soot-forming component.19 ATOFMS has been widely used in the study of atmospheric aerosols4 and emissions from the combustion of hydrocarbon fuels, such as diesel, petrol, and biofuels,20,22−25 but the technique

INTRODUCTION The combustion of biomass and hydrocarbon fuels is known to produce significant gaseous and particulate emissions (inorganics and soot). Very recently, it has been demonstrated that soot makes a much larger contribution to global warming than previously recognized;1 consequently, studying the mechanism of soot formation from biomass is of great importance, in order to minimize its emission. Moreover, soot particles are known to be inhalable carriers of highly carcinogenic compounds such as polycyclic aromatic hydrocarbons (PAHs).2−7 To reduce aerosol emissions by means of process-integrated primary measures, it is necessary to discover how particle properties are correlated with fuel composition and operating conditions. The molecular mechanism of the formation of carbonaceous soot during the combustion of mainly hydrocarbon fuels has been extensively studied, and there is general agreement concerning the involvement of radicals, usually resonancestabilized, in the formation of PAH either as soot precursors or as stable products. Many comprehensive reaction schemes have been proposed that incorporate the following individual routes to PAH: (i) hydrogen abstraction/carbon addition (HACA) via acetylene and/or butadiene radicals;8−10 (ii) reaction of cyclopentadienyl (CPDyl) radicals, originating from phenoxy produced by oxidation of aromatics or from phenols11,12 generating naphthalene and indene; (iii) production of higher PAH from naphthalene and indene by HACA or further reaction with CPDyl © 2013 American Chemical Society

Received: November 28, 2012 Revised: January 25, 2013 Published: January 29, 2013 1668

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

Article

An ATOFMS (TSI Model 3800) provides size (d = 100−3000 nm) and chemical composition information for individual particles, and it has been described previously in detail.34 It operates in a continuous sampling manner, allowing for real-time analysis of particles with high time resolution. First, the particles are focused into a narrow beam and transported with high efficiency into the ion source of the ATOFMS, using an aerodynamic lens system (TSI, Model 3800-100); the particles then are accelerated to a terminal velocity that is proportional to their aerodynamic diameter. The particles then enter a sizing region where they pass through two continuous-wave (CW) diode-pumped solid-state (DPSS) laser beams. Scattering signals are collected as each particle passes through each laser beam. This measures a particle timeof-flight (TOF) that can be converted to an aerodynamic size with a calibration curve. After being sized, the particle travels to the ion source region of a TOF mass spectrometer. Chemical species in the particle are desorbed/ionized, using the pulse output from an ultraviolet laser (Nd:YAG laser operating at 266 nm), and separated and detected using a reflectron TOF mass spectrometer equipped with multichannel plates (MCPs) for ion detection. The instrument records two mass spectra, one for each ion polarity, along with the observed particle size distribution. Mass calibration was undertaken using a solution of cesium iodide and sodium bromide, which allowed calibration up to m/z = +393. Since we were only extrapolating up to +500, and the peaks we are concentrating on are less than m/z = +400, this was considered to be a sufficiently accurate method. Mass spectrographs are generated in real time for each ablated particle analyzed and data logged for subsequent analysis. The data analysis technique used has been optimized to improve the average spectra produced. Previously, MS spectra were individually normalized by peak area prior to averaging, with no laser attenuation filters used during data collection; however, a more-representative average mass spectrum (MS) was obtained by using absolute peak areas and attenuation filters, so the MS spectra considered here have a laser energy of the mean ± 2σ and fewer hot spots within the beam. Examples of the flames are shown in Figure 1, and it is clear that there are significant variations in the amounts of soot produced at the

has found only limited application to products from the burning of biomass.26,27 Online use of ATOFMS avoids sample preparation and handling that can also give rise to surface oxidation, and it allows the examination of particulate matter from different stages of the combustion process. For example, Gao et al.28 compared PAH in coal soot by online UV photoionization TOFMS and gas chromatography−mass spectrometry (GC-MS) and found marked differences. Previous analyses of atmospheric aerosol and emissions from wood combustion have been carried out by Bente et al.,29 by means of two-step laser desorption ionization ATOFMS, and recently in a field study by Healy et al.30 using a commercial ATOFMS, similar to that used in this study. Moreover, a new study has been published31 on the use of ATOFMS for analysis of particles produced in combustion of wood to identify markers for organics, soot, and ash components; this also identified the nature of the particulates during the various modes of combustion. Studies have been carried out using aerosol mass spectrometry (AMS) and highmass-resolution AMS, for example.32 This last method is different from that used here, because AMS uses a thermodesorber instead of a laser to carry out particle desorption and, consequently, only the volatile fraction of soot can be analyzed. Very recently, a new instrument is available: the soot particle mass spectrometer (SP-AMS), which overcomes this problem and detects the refractory core quantitatively as well as the adsorbed organic species.33 These previous studies have analyzed complex soot samples produced from the combustion of real complex biomass samples; however, in this mechanistic study, we have taken the novel approach to burn individual components of wood to gain an insight into the mechanism of soot formation using ATOFMS. Previously, we have studied soot produced in the combustion of n-decane and eugenol,20 using a commercial ATOFMS, without sample dilution. In this paper, we broaden the study of these types of soots both by using a range of desorption ionization laser powers and by adding a dilution stage that allows us to derive particle size information (although not shown in this work). In addition, soot from the combustion of another liquid biomass proxy, furfural, has been studied to model cellulose, and compared to measurements for eugenol (a lignin model), and n-decane as a hydrocarbon for comparative purposes. As in our previous paper,20 a diffusion flame burner was employed because it is a convenient way to study the flames of liquids with high boiling points.

■

Figure 1. Flames of (a) n-decane, (b) furfural, and (c) eugenol. same height above the flame by the different fuels increasing in the following order: eugenol > furfural > n-decane. This is consistent with previous studies where sooting tendencies increase with the number of aromatic carbon atoms per molecule.21 The ratio EC/TC (elemental carbon/total carbon) can give an indication of total condensed organic species on the soot particles, and these were determined based on the work by Ferge et al.26 and as in our previous work.20

EXPERIMENTAL METHOD

Furfural, eugenol, and n-decane (with purities of 98%, 99%, and 99%, respectively; Sigma−Aldrich) were combusted in a wick burner as done previously,20 and the particulate emissions were sampled from the burner chimney for analysis by the ATOFMS. However, the experimental setup has been improved in relation to our previous work.20 Previously, the sample was directly introduced to the ATOFMS producing saturation of the sizing region of the instrument and preventing the acquisition of correct size information. To reduce particle concentration, a dilution system has been included; however, this resulted in a longer residence time of ∼85 s (previously 18 s), but allowed accurate size information to be obtained. Moreover, the extraction system has been developed to reduce the ingress of laboratory airborne particles. Isokinetic sampling from the chimney has been used in contrast with the previous work. In addition, attenuation filters to reduce variation in beam intensity have been used and, consequently, gave more representative averaged spectra.

■

EXPERIMENTAL RESULTS Typical experimental results obtained using the ATOFMS for n-decane, furfural, and eugenol are given in Figures 2−4 and show the effect of changing laser power on the positive mass spectra of the soots. Each figure shows the positive-ion mass spectra in two regions: m/z < 250 and region m/z > 250. Negative-ion mass spectra were also measured and show the presence of oxygenated species for 1669

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

Article

Figure 2. Normalized positive-ion mass spectroscopy (MS) spectra of n-decane soots at five laser desorption ionization (LDI) laser power densities. Other important peaks are observed at 102, 114, 228, 324, 352, 402. The difference between the different power levels can only be seen clearly in the online color version.

both furfural and eugenol and nitrogenated compounds for all compounds (m/z = −46, NO2−; m/z = −62, NO3−). The fact that the signal for m/z = −46 was, in all cases, more intense than that for m/z = −62, indicating the presence of nitro-PAH, as previously discussed by us.19 Nevertheless, since this work is not focused on heteroatoms and negative mass do not aid PAH identification, the negative-ion mass spectra are not shown here. With the exception of masses >250 in the eugenol soot spectrum, there are significant similarities using high laser power

density between the positive-ion mass spectra of the soots from all three fuels. At the highest laser power density, no fuel-to-fuel differences could be identified and the most intense peaks are attributed to carbon clusters, Cn+ (n = 1 −6). At lower power densities, the mass spectra are again dominated by these peaks but with n = 3−16. Peaks are also seen at m/z values resulting from the fragmentation of large molecules. By reducing the laser power even more, we can start to see molecular ion peaks and differences between the three types of soot are accentuated. 1670

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

Article

Figure 3. Normalized positive-ion MS of furfural soots at five laser desorption ionization (LDI) laser power densities. Other important peaks are observed at 102, 114, 128, 228, 347, 402. The difference between the different power levels can only be seen clearly in the online color version.

Peaks detected when a laser power density of 1.8 × 107 W cm−2 was used for ablation of the particles are summarized in Table 1, together with ions formed from PAH molecules. The assignment in the ATOFMS is quite challenging, since fragmentation occurs even at the lowest power. The proposed assignments in Tables 1 and 2 are the more likely identifications backed up to an extent by evidence from model compound work and modeling. The molecular structures assigned, when possible, to each ion are displayed in Table 2.

Furfural Mass Spectra. At a laser power density of 1.8 × 107 W cm−2 for ablation of the particles, the furfural soot spectrum contains two overlapping sequences, each consisting of peaks with a progressive increase of 14 amu. Peaks from the furfural soot at m/z +178, +192, +206, and +220 are consistent with a series of 3-ring PAHs, the alkylated phenanthrenes/ anthracenes. The other apparent progression of peaks also consists of peaks separated by 14 amu from +174 to +216, of which +202 and +216 can be assigned to pyrene/fluoranthene (4-ring PAH) and their methyl derivatives. 1671

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

Article

Figure 4. Normalized positive-ion MS of eugenol soot at five laser desorption ionization (LDI) laser power densities. The difference between the different power levels can only be seen clearly in the online color version.

Eugenol Mass Spectra. A series of peaks, separated by 12 units, were found for all three fuels (although the series starts at different m/z for different fuels) namely, 90, 102, 114, 128 (only in furfural), 140, 152, 164, 176, 188, 200, 212 (only in eugenol), and 224 and 236 (both very weak for n-decane and furfural and intense for eugenol). The peaks are more intense in eugenol, because of the higher carbon content. The peaks m/z = 128 and m/z = 152 can be identified as originating from naphthalene and acenaphthylene, respectively; the remainder may be a series resulting from a MAC and HACA sequence.

The actual identity of 200, 212, 224, and 236 is obscure, but oxygen-containing compounds and/or fragment ions may be considered. An additional peak that can be observed for all three fuels is m/z = 228 and corresponds to molecular ions for chrysene, benz[a]anthracene, and triphenylene. Species with m/z = 228 could be taken into account via the addition of butadiene to phenanthrene. A significant difference between the positive-ion mass spectra of soots from the three fuels at low laser power densities is the intense peak at +39, seen with high intensity only for furfural 1672

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

Article

Combustion Model Calculations. In parallel with the experimental study, a study of the oxidation and PAH formation from n-decane was made using the plug flow and opposed diffusion flame models (PLUG-FLOW and OPPDIF programs, respectively) from Chemkin-Pro,35 and using the CRECK36,37 hydrocarbon oxidation mechanism. The computation of species formation and flow field is difficult in this type of burner, but it is possible to use Chemkin-Pro in order to determine the possible reaction products. In the first case considered, it was assumed that the reaction zone extended from just within the tip of the flame and extended into the burned gas region. Computations using the PLUG-FLOW program were made on the assumption that the flame is partially premixed with air and using the CRECK mechanism to calculate the concentration of the species formed from n-decane during combustion at a typical reaction zone temperature (1850 K). The amount of oxygen was varied, but this only changed the amount of PAH formed and not the relative composition of the PAH components; a ratio of {5 mol % O2 to 1 mol % n-decane} was found to be appropriate. The species formed under these conditions are shown in Figure 5. In common with many other studies, this shows that

Table 1. Summary of the Likely Assignment of the Peaks between 128 Da and 250 Da Seen in Soot from the Combustion of the Fuels, Analyzed at a DI Laser Power Density of 1.8 × 107 W cm−2 (the Most Intense Peak in This Region is Underlined for Each Fuel) n-decane

eugenol

+152

+152

+203

+203

furfural

assignmenta

+128 +152 +178 +192 +202 +206

naphthalene acenaphthylene C0 3-ring PAH C1 3-ring PAH C0 4-ring PAH C2 3-ring PAH

+216 +220

C1 4-ring PAH C3 3-ring PAH

+228

C0 4-ring PAH

+212 +217

+228 a

+224 +228 +248

PAH = polycyclic aromatic hydrocarbon.

soot (for other fuels, it was a minor peak), and assigned to C3H3+ (since potassium is excluded after investigating the isotopic ratio for 39:41 in the mass spectra). Moreover, at low laser power density, the n-decane and eugenol soot contain several peaks with masses 1 amu greater than the corresponding peaks observed for furfural soot. This would be consistent with deprotonation in furfural. A large difference is seen at m/z > +250, in that a series of peaks separated by ∼12 amu is evident in the spectrum of eugenol soot, attributed to a sequence of ions containing increasing numbers of C atoms (recall Figure 4). From the positive-ion spectra, it was concluded in relation to PAH content that (1) n-decane soot contains mainly PAH with four-ring structures and their alkyl derivatives; (2) eugenol soot PAH were similar but with a series of peaks separated by 12 and 24 Da extending beyond 350 Da; and (3) furfural soot mainly contained both three- and four-ring PAH with their alkyl derivatives. Comparing these results with our previous off-line measurements,18,19 where the entire sample is collected (at 40 °C, using a 1.2-μm quartz filter), it is clear that the contribution of species up to approximately two-rings is smaller on the particles, presumably in the present experiments, because of the temperature in the sampling system and evaporation. EC/TC Ratios. The ratio of elemental carbon to total carbon (EC/TC) was determined at different laser powers to assess the variation in composition through the soot particles. At low power only, the surface of the particles is analyzed, but at high power, the entire particle is ablated. Only small differences are observed for the three fuels, and it appears that differences between the soots are only minor both at the surface and inside the particle. The calculated EC/TC values were ∼40% for the soot produced by combustion of the three fuels using a laser power corresponding to 3 × 107 W cm−2, by applying the method of Ferge et al.26 At a higher laser power, the values tended toward 90%, indicating that it does not have enough power in the later laser ablation to convert all molecules to elemental ions. In our previous work,20 we used a laser power of 3 × 107 W cm−2 and EC/TC ratios of 52.6% and 88.5% were obtained for eugenol and n-decane, respectively. The differences can be explained by the use of a different sampling arrangement, which influences EC/TC ratios, because of the longer residences times during sampling and the difference in data treatment. A detailed study of how different factors can affect these ratios will be presented elsewhere.

[1‐ring aromatic compounds, benzene, toluene, cresol, etc.] ≫ [2‐ring, naphthalene, etc.] ≫ [3‐ring, anthracene, etc.]

and so on, and these results are similar to those obtained experimentally in the combustion of wood or diesel fuels.2,4,17 In the second approach considered, computed results were obtained using the Chemkin-Pro opposed diffusion flow model (OPPDIF). This was applied to the wick flame at a position approximately halfway up the wick and the opposing gas flows were taken to be 3 cm/s for n-decane and 10 cm/s for air, both at 500 K. The results are shown in Figures 6a−d as follows: (i) PAH are formed on the rich side of the diffusion flame, as expected, and the ratios of the PAH compounds are similar to those observed in Figure 5; and (ii) there is a difference between the computed gas-phase results and the experimental values arising from the surface composition of soot; the former are similar to many studies where all the aromatic species are collected, but this is dependent on the sampling conditions. The ATOFMS experimental results at low laser power relate to the surface “attached” species and at higher laser power, where ablation takes place, to subsurface species, which are mainly carbon. These species will play a direct role in the growth of the soot particle; however, their abundance in the aerosol will be limited by their volatility and consequent losses in the aerodynamic lens sampling system in the ATOFMS. In contrast, higher (growing) PAH species will be more abundant in the experimental MS spectra. In conclusion, there are differences between the experimental and calculated data, in that the ATOFMS results relate to the soot surface (or at higher laser levels the subsurface), whereas the calculated data show that, in the flame, the concentration of the soot-forming species is higher than that at the surface. Consequently, the soot concentration of PAH in the flame is higher than that in the gas phase and gives some information on the soot growth mechanism. 1673

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

Article

Table 2. Suggested Molecular Structures of PAH Responsible For Ions Seen In Soot Mass Spectra

4. DISCUSSION

wood soot mass spectrum. This was attributed to the release of eugenol and, hence, soot formation from the pyrolysis of lignin. We also postulated that cellulose and hemicellulose form levoglucosan, galactose, and mannose via the thermal

In our previous studies,19,20 it was shown that the eugenol soot mass spectrum had characteristics similar to those of the pine 1674

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

Article

Figure 5. Computed pyrolysis product for n-decane, assuming premixed combustion under rich conditions.

decomposition of pine wood and that they largely subsequently decompose to CO and H2 and do not contribute significantly to soot formation. However, some traces of these compounds can escape and are used as biomarkers.5 However, furfural is formed from the polysaccharide hemicellulose, especially at high heating rates,38 and little is known about the fate of this compound; the evidence given here does suggest that it has the capability of forming some soot. However, the contribution of the different routes of PAH and soot formation must be different from those in eugenol as the differences in the soot MS spectrum indicate (see Figures 3 and 4). In the case of n-decane, varying contributions from several mechanisms may be invoked to account for similarities and differences.30 Possible routes to the soot precursor PAH include the following: (i) cyclopentadienyl (CPDyl, and further reaction of CPDyl, although the initial products (indene and naphthalene) are not detected, since they are more volatile and may be lost in the aerodynamic lens system of the ATOFMS, but mainly in the dilution system;39 (ii) hydrogen abstraction carbon addition (HACA); and (iii) reaction with other PAH-derived radicals. A reaction with methyl may provide a route either to yield methyl PAH derivatives or to bridge “bay” positions by methyl addition/cyclization. These routes are all seen in opposed diffusion flame studies (using OPPDIF) of n-decane combustion and the outcome is shown in Figure 6. Figure 6a shows the temperature profile through the reaction zone. Figure 6b shows the reaction of some of the major species and Figure 6c shows the formation of some of the major PAH species and includes an indication of the soot formation, which is based on the dimerization of pyrene and is defined as BIN 1A in the program used (the concentration is given as a mole fraction). It should be noted that the computed flame temperature is not corrected for radiation from the soot and will be higher than that from a real flame. A calculation for a similar flame in which the fuel was diluted with 10% N2 showed that this did not influence the relative concentrations of the PAH species. Detailed analysis of the rates of production using the Chemkin-Pro subroutine shows how the contributions from the different routes vary throughout the flame zone, as might be expected. Near the fuel input at ∼500 K, the initial breakdown is to the decyl n-C10H21 radical and its decomposition products (such as alkenes); PAH are formed, but only via a CPD mechanism. Further into the hot reaction zone, acetylene and HACA become important. A scheme for the overall formation of PAH and, hence, soot is given in Figure 7. Furfural is a cyclic compound with a C/H ratio higher than n-decane and, therefore, a potentially higher sooting tendency

Figure 6. (a) Temperature profile of the opposed diffusion flame as a function of the distance between the inlet streams of n-decane (LHS) and air (RHS). (b) Concentration profiles of some major species: O2 (curve 1); CO2 (curve 2); CO (curve 3); C2H2 (curve 4); and C6H6 (curve 5). (c) Concentration profiles of PAH species and soot (Bin 1A). 1675

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

Article

Figure 7. Diagram showing the PAH formation paths in the three flames.

might be expected,21 since on-ring fission of the radical produced in the initial reaction both CO and the propargyl radical (C3H3·)39 are formed as follows:

to soot formation.11,40 These products are consistent with the mass spectra observed for soot formed by the combustion of these fuels (cf. Figure 3). There is strong evidence of m/z = 39 due to C3H3+, probably from fragmentation of PAHs formed by propargyl (C3H3·) addition, which is consistent with the mechanism above and that is not observed for other sooting fuels. Once propargyl is formed, it can react with acetylene, formed during furfural oxidation,41 to yield CPDyl and then, as discussed above, indene and naphthalene. This is consistent with the observation of the peak at m/z = 39 during the combustion of furfural but not for the others. Methyl and dimethyl PAH in the MS analysis of furfural soot might indicate the operation of a channel in which methyl radicals combine with PAH radicals. Eugenol is an allyl chain-substituted guaiacol, 4-allyl-2methoxyphenol, and it is a much more complex molecule. Although it can be used as a diesel bioadditive,42 little is known about the kinetics of the thermal decomposition routes at flame temperatures, although there is considerable information related to thermal processing.43,44 There is some evidence that it would produce the allyl radical (C3H5·) and it is likely that this radical and the 2-methoxyphenol cyclic radical would both decompose to give acetylene, which would explain the nature of the mass spectra shown in Figure 4, which is consistent with HACA growth.

and then by

An alternative to reaction 1 is reaction 3:

followed by

and then reaction 2. The enthalpies of reaction for reactions 1−4 were calculated using MOPAC, and they indicate the thermodynamic preference of reaction 3 over reaction 1. When the full reaction scheme is considered, reactions 1 and 2, taken together, are more favorable, compared with reactions 3 and 4 together, which is why we see indications of C3H3 chemistry. Propargyl plays an important role in the soot-forming reactions, especially in the formation of benzene and the route 1676

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

■

and C3H5·

=

CH3·

+ C2 H 2

which reacts further to give products including C2H 2

REFERENCES

(1) Bond, T. C.; Doherty, S. J.; Fahey, D. W.; Forster, P. M.; Berntsen, T.; et al., J. Geophys. Res.: Atmos. 2013, 118, DOI: 10.1002/ jgrd.50171. (2) Crutzen, P. J.; Andreae, M. O. Science 1990, 250, 1669−1678. (3) Denissenko, M. F.; Pao, A.; et al. Science 1996, 274, 430−432. (4) Laskin, A.; Laskin, J.; Nizkorodov, S. A. Environ. Chem. 2012, 9, 163−189. (5) Williams, A.; Jones, J. M.; Ma, L.; Pourkashanian, M. Prog. Energy Combust. Sci. 2012, 38, 113−137. (6) Orasche, J.; Seidel, T.; Hartmann, H.; Schnelle-Kreis, J.; Chow, J.; Ruppert, H.; Zimmermann, R. Energy Fuels 2012, 26, 6695−6704. (7) Furuhata, T.; Kobayashi, Y.; Hayashida, K.; Arai, M. Fuel 2012, 91, 16−25. (8) Frenklach, M.; Wang, H. In Soot Formation in Combustion: Mechanisms and Models; Bockhorn, H. Ed.; Springer−Verlag: Berlin, 1994; pp 162−192. (9) Colket, M. B.; Hall, R. J. In Soot Formation in Combustion: Mechanisms and Models; Bockhorn, H. Ed.; Springer−Verlag: Berlin, 1994; pp 442−470. (10) Richter, H.; Howard, J. B. Phys. Chem. Chem. Phys. 2002, 4, 2038−2055. (11) D’Anna, A.; Violi, A. Energy Fuels 2005, 19, 79−86. (12) Cypres, R.; Bettens, B. Tetrahedron 1974, 30, 1253−1260. (13) Violi, A.; Sarofim, A. F.; Truong, T. N. In Proceedinds of the Second Mediterranean Combustion Symposium, Sharm El-Sheik, Egypt, Jan. 2002; pp 1161−1170. (14) Juntgen, H. Erdoel Kohle, Erdgas, Petrochem. 1985, 83, 448−455. (15) Fetzer, J. C.; Biggs, W. R. Polycyclic Aromat. Compd. 1994, 4, 3− 17. (16) Shukla, B.; Koshi, M. Combust. Flame 2011, 158, 369−375. (17) 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. (18) Fitzpatrick, E. M.; Ross, A. B.; Bates, J.; Andrews, G. E.; Jones, J. M.; Phylaktou, H.; Pourkashanian, M.; Williams, A. Process Saf. Environ. Protect. 2007, 85 (5), 430−440. (19) Fitzpatrick, E. M.; Jones, J. M.; Pourkashanian, M.; Ross, A, B.; Williams, A.; Bartle, K. D. Energy Fuels 2009, 22, 3771−3778. (20) Baeza-Romero, M. T.; Wilson, J. M.; Fitzpatrick, E. M.; Jones, J. M.; Williams, A. Energy Fuels 2010, 24, 439−445. (21) Bartle, K. D.; Fitzpatrick, E. M.; Jones, J. M.; Kubacki, M. L.; Plant, R.; M. Pourkashanian, M.; Ross, A. B.; Williams, A. Fuel 2011, 90, 1113−1119. (22) Gross, D. S.; Gälli, M. E.; Silva, P. J.; Wood, S. H.; Liu, D. Y.; Prather, K. A. Aerosol Sci. Technol. 2000, 32, 152−163. (23) Gross, D. S.; Barron, A. R.; Warren, B. S.; Sukovich, E. M.; Jarvis, J. C.; Suess, D. T.; Prather, K. A. Atmos. Environ. 2005, 39, 2889. (24) Dutcher, D. D.; Pagels, J.; Bika, J.; Franklin, L.; Stolzenburg, M.; Thompson, S.; Medrano, J.; Brown, N.; Gross, D. S.; Kittelson, D.; McMurry, P. H. Atmos. Environ. 2011, 45, 3406−3413. (25) Toner, S. M.; Sodeman, D. A.; Prather, K. A. Environ. Sci. Technol. 2006, 40, 3912−3921. (26) Ferge, T.; Karg, E.; Schröppel, A.; Coffee, K. R.; Tobias, H. J.; Frank, M.; Gard, E. E.; Zimmermann, R. Environ. Sci. Technol. 2006, 40, 3327−35. (27) Healy, R. M.; Hellebust, S.; Kourtchev, I.; Allanic, A.; O’Connor, I. P.; Bell, J. M.; Healy, D. A.; Sodeau, J. R.; Wenger, J. C. Atmos. Chem. Phys. 2010, 10, 9593−9613. (28) Gao, S.; Zhang, Y.; Li, Y.; Meng, J.; He, H.; Shu, H. J. Int. J. Mass Spectrom. 2008, 274, 64−69. (29) Bente, M.; Sklorz, M.; Streibel, T.; Zimmermann, R. Anal. Chem. 2008, 80, 8991−9004. (30) Healy, R. M.; Sciare, J.; Poulain, L.; Kamili, K.; Merkel, M.; Müller, T.; Wiedensohler, A.; Eckhardt, S.; Stohl, A.; Sarda-Estève, R.; McGillicuddy, E.; O’Connor, I. P.; Sodeau, J. R.; Wenger, J. C. Atmos. Chem. Phys. 2012, 12, 1681−1700.

(6)

(8)

where C2H2 and CPDyl (cyclopentadienyl) are major species in the soot-forming mechanism. Unfortunately, kinetic data are not available to model the opposed diffusion flame of furfural and eugenol at this stage, but the suggested routes are shown in Figure 7. Computations using the OPPDIF program for phenol, anisole, or toluene as a fuel under similar conditions as those described for the n-decane show that the PAH products are broadly similar in nature to those for n-decane, although there are differences in the relative concentrations. However, the concentrations of PAH and soot in the flame zone are highest for the toluene flame with those for phenol and anisole intermediate between n-decane and toluene. This would parallel the sequence observed in the present work with aromatic compound eugenol, producing more soot than the oxygenated compound furfural, which formed more soot than n-decane.

■

CONCLUSIONS The diffusion flames of three fuelsn-decane, furfural, and eugenolhave been studied using a wick flame burner. The first fuel represents a typical hydrocarbon fuel; the other two fuels are typical wood pyrolysis products, from cellulose and lignin, respectively. The sooting tendency in these three fuels has the following ranking/sequence: eugenol > furfural > n-decane. From these single-component studies, it seems that cellulose (studied through furfural) can contribute to soot formation, together with lignin (studied using eugenol). An investigation of the combustion of binary mixtures would be desirable. Combustion modeling for an opposed diffusion flame of n-decane was made, and the results aided the interpretation of the mass spectrometry (MS) results for the other fuels, in order to generalize the results. The characteristic mass spectra of the three fuels studied were obtained using the aerosol time-of-flight mass spectrometer (ATOFMS). These showed significant differences, which result from the different contributions of the soot formation routes for the different fuels. There are strong indications of HACA and MAC series for three fuels; and probably cyclopentadienyl (CPDyl) reactions, which, along with HACA, would provide a route to phenanthrene/anthracene (m/z = 178), pyrene (m/z = 202), and chrysene (m/z = 228), etc. Channels for soot formation for different fuels are summarized in Figure 7.

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ⊥

Pacific Northwest National Laboratory, Richland, WA, USA.

Notes

The authors declare no competing financial interest. 1677

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678

Energy & Fuels

Article

(31) Pagels, J.; Dutcher, D. D.; Stolzenburg, M. R.; McMurry, P. M.; Gaelli, M. E.; Gross D. S. J. Geophys. Res.: Atmos. 2013, DOI:10.1029/ 2012JD018389. (32) Schneider, J.; Weimer, S.; Drewnick, F.; Borrmann, S.; Helas, G.; Gwazec, P.; Schmid, O.; Andreae, M.,O.; Kirchner, U. Int. J. Mass Spectrom. 2006, 258, 37−49. (33) Onasch, T. B.; Trimborn, A.; Fortner, E. C.; Jayne, J. T.; Kok, G. L.; Williams, L. R.; Davidovits, P.; Worsnop, D. R. Aerosol Sci. Technol. 2012, 46, 804−817. (34) Gard, E.; Mayer, J. E.; Morrical, B. D.; Dienes, T.; Fergenson, D. F.; Prather, K. A. Anal. Chem. 1997, 69, 4083−4091. (35) Reaction Design, Inc. www.Reactiondesign.com, accessed Jan. 10, 2013. (36) Ranzi, E.; Frassoldati, A.; Granata, S.; Faravelli, T. Ind. Eng. Chem. Res. 2005, 44, 5170−5183. (37) Ranzi, E.; Faravelli, T. www. creckmodeling.chem.polimi.it/ kinetic.html, accessed Jan. 24, 2012. (38) Greenhalf, C. E.; Nowakowski, D. J.; Harms, A. B.; Titiloye, J. O.; Bridgwater, A. V. Fuel 2012, 93, 692−702. (39) Bente, M.; Martin Sklorz, M.; Streibel, T.; Zimmermann, R. Anal. Chem. 2008, 80, 8991−9004. (40) Liu, F.; Dworkin, S. B.; Thomson, M. J.; Smallwood, G. J. Combust. Sci. Technol. 2012, 184, 966−979. (41) Thorton, M. M.; Malte, P. C.; Crittenden, A. L. Proc. Combust. Inst. 1986, 21, 979−989. (42) Kadarohman, A.; Hernani.; Rohman, I.; Kusrini, R.; Astuti, R. M. Fuel 2012, 98, 73−79. (43) Amen-Chen, C.; Pakdel, H.; Roy, C. Bioresour. Technol. 2001, 79, 277−299. (44) Ledesma, E. B.; Campos,C.; Cranmer, D. J.; Foytik, B. L.; Ton, M. N.; Dixon, E. A.; Chirino, C.; Batamo, S.; Roy, P. Energy Fuels 2013, DOI: 10.1021/ef3018332.

1678

dx.doi.org/10.1021/ef3019386 | Energy Fuels 2013, 27, 1668−1678