Article pubs.acs.org/EF
Chemical Features of Particles Generated in an Ethylene/Ethanol Premixed Flame Mariano Sirignano,† Anna Ciajolo,‡ Andrea D’Anna,*,† and Carmela Russo‡ †
Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy ‡ Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche (CNR), Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy ABSTRACT: An ethylene\ethanol premixed flame has been investigated to elucidate the effect of ethanol addition on particle morphology and chemical features in controlled flame conditions. In situ optical techniques and ex situ online particle size distribution measurements showed that soot formation is strongly reduced for the effect of ethanol addition to ethylene and that soot particle size is much lower with respect to the size of the soot particles formed in a neat ethylene flame operated under the same combustion conditions. It was also noticed that the formation of nanoparticles with sizes smaller than 10 nm is not significantly affected by ethanol addition to ethylene; their concentration remains practically unchanged or even increases for the effect of ethanol addition. Particles produced in the ethylene/ethanol flame showed a higher reactivity with respect to the ethylene-generated particles. Raman spectra indicated that the presence of ethanol inhibits the aromatization process, showing, on average, a smaller size of the aromatic island within the particles. Ultraviolet−visible analysis confirmed this finding, showing a marked molecular character for the particles collected in the ethylene/ethanol flame with respect to the neat ethylene flame, especially for the particles with sizes smaller than 20 nm. Finally, Fourier transform infrared spectra evidenced the presence of oxygen functionalities onto the particles formed when ethanol is used; the molecular character and bonded oxygen atoms can contribute to the major reactivity of the ethanol-generated particles, as also evidenced by thermogravimetric analysis.
■
carbon.6 Most of the studies on the relation between the soot nanostructure and biofuel characteristics6,8,9,11 have been conducted on practical combustion systems, such as engines and furnaces. The lab-scale combustion devices and particularly premixed flames, in which fluid-dynamic effects are separated from kinetic effects, allow for better control of the combustion parameters, leading to more general conclusions about the link between the biofuel structure and the chemical features of the emitted particles. Among biofuels, ethanol is one of the most used oxygencontaining additives in combustion because it can be obtained from biomasses at a reasonable cost.16 Different studies have been performed aimed at understanding the role of ethanol on combustion features and particulate emission. Studies performed in shock-tube pyrolysis experiments17,18 and premixed flames19−23 have shown a beneficial effect of ethanol addition on bulk carbon particulate emission; the addition of ethanol reduces the amount of aromatics, including benzene and polycyclic aromatic hydrocarbons (PAHs), and soot particles. Salamanca et al.23,24 have found that particles generated by ethanol-doped premixed ethylene flames are mainly in the nanometer size range; most of them have sizes well below 10 nm. These latter particles do not contribute significantly to the mass of the particulate emitted but dominate the number
INTRODUCTION Biofuels produce less net carbon dioxide emissions than oilbased conventional fuels and are commonly considered able to reduce the engine-out emissions.1−3 Among biofuels, alcohols are considered very important, especially for fueling internal combustion engines. Besides the beneficial effect of alcohol blending on the amount of particulate matter formed, it is also important to evaluate the effect on the chemical and morphological features of the bulk particulate matter, which are known to affect its toxicity.4 Overall, one of the peculiar characteristics of biofuels is the presence of one or more oxygen atoms chemically bonded to the C atoms. The presence of oxygen-bonded atoms within the hydrocarbon structure may significantly change the main oxidation and molecular growth pathways.5 Indeed, reaction pathways not active during hydrocarbon combustion can be activated or enhanced by oxygen-borne atoms, leading to the formation of species with high molecular mass possibly still having oxygen atoms bonded to the structure.6−13 Moreover, these compounds can participate in the formation of the soot particles and can be found in the soot aggregates emitted from combustion systems. Previous studies on the nanostructure of particles produced when using biofuels have evidenced significant changes in the particle characteristics in particular, an enhanced oxidation in the after-treatment systems14 as well as a decrease of the particle size,13 with this latter point raising concerns on human health.15 The enhanced reactivity of biofuel-derived soot particles has been linked to oxygen bonded to the soot surface,9 the particle size,13 and/or the content of amorphous © 2016 American Chemical Society
Special Issue: In Honor of Professor Brian Haynes on the Occasion of His 65th Birthday Received: September 15, 2016 Revised: December 20, 2016 Published: December 27, 2016 2370
DOI: 10.1021/acs.energyfuels.6b02372 Energy Fuels 2017, 31, 2370−2377
Article
Energy & Fuels
comparison of the sooting tendency and chemical signatures of the generated particles. Temperature measurements were performed along the flame axis by a Pt/Pt−Rh thermocouple using a fast insertion procedure to avoid particulate deposition on its bead to verify that the experiments were performed in similar temperature conditions. In Situ Optical Measurements. The fourth harmonic of a Nd:YAG pulsed laser at 266 nm was used as an excitation source for spectrally resolved laser-induced emission (LIE) measurements. Signals were collected by an intensified charge-coupled device (CCD) camera, thermoelectrically cooled to −10 °C to reduce noise, with a gate time of 100 ns, coupled to a spectrometer to spectrally resolve the LIE signal in the 200−550 nm range. Both laserinduced fluorescence (LIF, between 300 and 450 nm) and laserinduced incandescence (LII, at 550 nm) signals were simultaneously detected. Dependent upon the flame structure and operating conditions, two maxima could be distinguished in the fluorescence spectra. The first one is located in the spectral region between 320 and 380 nm, i.e., the near UV region, and the second one is in the spectral region between 380 and 450 nm. LIF has been widely used to track species in flame and successively applied to study the evolution of combustion byproducts ranging from gas-phase PAHs to condensed phase aromatic species.34,35 LIF in the UV region measured in premixed flames has been in fact attributed to nanosized particles made up of aggregates of PAHs having two-ring aromatic subunits connected by aliphatic or, possibly, oxygen bonding rather than single, gas-phase PAHs.36−39 The rise of fluorescence in the visible region has been associated with a progressive aromatization of the precursor structure in their transformation to soot nuclei.36−39 More recently, the attribution of LIF to nanoparticles has been confirmed by timeresolved fluorescence measurements, under the same flame conditions of the present study, showing a significantly long decay time with respect to PAHs.40−42 The LII signal appeared as a continuum in the visible region, and it has been attributed to solid soot particles, which are able to dissipate the acquired energy by thermal emission rather than LIF. Incandescing particles as small as 5−10 nm can be detected in flames; however, the largest contribution to the LII signal is given by soot particles larger than 10 nm and by aggregates up to 100 nm.40−44 Particulate Matter Measurement. Particles were sampled along the flame axis for size distribution measurements using a stainless-steel probe placed horizontally in the flame. The probe has an internal diameter of 8 mm, a wall thickness of 0.5 mm, and a pinhole diameter of 0.8 mm, which is larger than those generally used in other works.25,42,45 The large pinhole has the advantage of reducing the clogging of the hole, a problem often encountered in fully sooting flames when smaller pinholes are used. Although the perturbation of the flame for the effect of the probe and the large pinhole was evident, the system has proven to be reliable for the determination of the particle size distributions (PSDs), even in fully sooting flames with equivalence ratios as high as 2.46.25,42,45 With the large pinhole, the dilution ratio in the probe line decreases and the possibility of coagulation of particles and condensation of gas-phase PAHs onto the particle surface increases. To reduce these two phenomena, the probe line was modified to obtain a two-stage dilution. The first dilution occurs in the probe itself using a carrier gas, similar to other apparatuses,24 while the second dilution stage is placed 10 cm after the pinhole, just before a differential mobility analyzer (DMA). The carrier gas flow rate was 1 NL/min in the probe, while the second stage dilution was obtained with 45 NL/min. The total residence time of the particles in the sampling line was 160 ms. The reliability of the twostage dilution procedure was proven by measuring the size distribution of the particles formed in slightly sooting flame conditions, where clogging of the pinhole did not occur. Comparable results were obtained using the 0.8 and 0.3 mm pinhole probes. Further details, comparative results, and a picture of the experimental apparatus can be found in previous works.42,45 For the PSD measurements, a nano-DMA was used (TapCon 3/ 150 DMA system in high-voltage mode, corresponding to a nominal size range of 2−100 nm, equipped with a Faraday cup electrometer detector). At the entrance of the DMA, a radioactive (Am-241) bipolar
concentration distribution of the emitted particles. Also, in comparison to the particle size distributions of ethanol-doped and neat ethylene flames, it has been found that, in slightly sooting flames, the addition of ethanol does not significantly affect particle coagulation, suggesting that the main effect relies on the main oxidation pathways that lead to the formation of particle precursors. Moreover, the presence of ethanol induces some modifications in the photoionization energy and presumably in the chemical features of the smaller particles.25 In diffusion flames, the amount of ethanol added strongly influences particle production. Small amounts of ethanol added to the fuel enhance the pyrolysis of the fuel mixture because of the presence of oxygen-containing radicals deriving from ethanol decomposition. The larger production of ethylene and methyl radicals allows for the molecular growth pathways that lead to the formation of aromatics and soot.26,27 When larger amounts of ethanol are added to the fuel mixture, ethanol pyrolysis prevails, leading to the formation of partially oxidized species, with the net effect of a reduction of the particle precursors and, lately, of the total mass of particulate matter.4,28,29 It is worth noting that, also in the diffusioncontrolled conditions investigated at lab scale, the reducing effect of ethanol addition on the formation of the smaller size particles is overall less evident than the reduction of the total particulate matter. The concentration of the smaller size particles is not significantly reduced, and in some cases, it is even increased by the addition of ethanol.24 Also, it appears that the oxidation and pyrolysis of ethanol as well as the environment in which it reacts have an impact on the amount, morphology, and chemical characteristics of the formed particles.24,25 In this work, nanostructural properties of the soot produced in atmospheric-pressure-rich premixed flames burning ethylene and ethylene/ethanol mixtures in similar combustion conditions are compared. The total mass concentration and particle size distributions as well as the aromatic content and oxygen functionalities of the combustion-formed particles are analyzed. In particular, thermogravimetric analysis (TGA), size-exclusion chromatography (SEC), and Raman, Fourier transform infrared (FTIR), and ultraviolet−visible (UV−vis) spectroscopies30−33 are used to retrieve information on morphological and chemical features of combustion-generated particles and to highlight the impact of ethanol addition on the particle structure.
■
EXPERIMENTAL SECTION
Flame Burner and Operating Conditions. The experimental setup is the same used by Salamanca et al.23,24 Stainless-steel capillary tubes packed in a 5.8 cm circular burner head are used to feed, at atmospheric pressure, an ethylene/air premixed flame with an equivalence ratio of 2.46 and a cold gas velocity of 10 cm/s, at standard conditions. A steel plate positioned at 3 cm from the burner exit is used to stabilize the flame. The burner head is temperaturecontrolled, with 1 L/min recirculating water kept at 70 °C with a thermostatic bath. This neat ethylene flame constituted our reference case. The ethylene flame doped with ethanol, hereafter named ethylene/ EtOH flame, was obtained by substituting part of ethylene with ethanol (20% of the total carbon was fed as ethanol), thus simulating a fuel composition that is typical for practical real combustion.1 Ethanol was fed to the preheated fuel stream by a syringe pump to control the flow rate. Preheating of the fuel stream was adopted also for the reference case (neat ethylene flame) to maintain the same boundary conditions in the two investigated flames. The adiabatic temperature of the ethylene/EtOH flame was close to the adiabatic temperature of the ethylene flame, which allows for a direct 2371
DOI: 10.1021/acs.energyfuels.6b02372 Energy Fuels 2017, 31, 2370−2377
Article
Energy & Fuels diffusion charger is placed, so that particles can reach Fuchs’ steadystate charge distribution.46 The PSDs obtained by the DMA were successively corrected for losses in the pinhole and probe following the procedure reported in the literature.47,48 The DMA separates particles on the basis of their mobility diameter, so that particle diameter was retrieved from the correlation proposed by Singh et al.49 Bulk particulate for the chemical characterization was collected thermophoretically on a 75 × 25 × 1 mm quartz plate used as a substrate to deposit particles. Particles were sampled by inserting the quartz plate horizontally in the flame for 2 s. This procedure was repeated several times, allowing for a cooling cycle at room temperature of 10 s after each insertion. The procedure was repeated to achieve an amount of material suitable for the structural analysis. The sampling procedure has been developed and recently used to perform structural analysis on the particulate matter collected in an ethylene/2,5-dimethylfuran flame.50 Tests have been performed with different numbers of insertions, showing no effect of the sampling procedure on the structure of the sampled material. Chemical Analysis of Particulate Matter. Particulate powder, mechanically ablated from the quartz plates, was analyzed by diagnostic techniques to give information on its chemical structure and composition and to assess its reactivity. Raman spectra were measured by means of a Horiba XploRA Raman microscope system (Horiba Jobin Yvon, Japan) equipped with a frequency-doubled Nd:YAG solid-state laser (λmax = 532 nm, 25 mW). More details on the Raman measurements and spectra acquisition are available elsewhere.50 FTIR spectra in the 3400−600 cm−1 range were acquired in the transmittance mode using a Nicolet iS10 spectrophotometer. Analyses were performed on sample dispersions prepared by mixing and grinding the samples in KBr pellets (0.2−0.3 wt %). More details on the sample preparation for FTIR measurements are available elsewhere.50,51 UV−vis absorption spectra of the particulate suspended in N-methyl-2-pyrrolidinone (NMP, with a concentration of 10 mg/L) were measured in a 1 cm quartz cell using an Agilent UV−vis 8453 spectrophotometer. The UV−vis spectra were measured also on the soot fraction of
