Study of carbon and carbon–metal particulates in a canola methyl ester air-flame

Combustion and Flame 162 (2015) 216–225

Contents lists available at ScienceDirect

Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e

Study of carbon and carbon–metal particulates in a canola methyl ester air-flame Wilson Merchan-Merchan ⇑, Henry O. Tenadooah Ware School of Aerospace and Mechanical Engineering, University of Oklahoma, 865 Asp Ave., Felgar Hall, Norman, OK 73019, USA

a r t i c l e

i n f o

Article history: Received 21 April 2014 Received in revised form 3 July 2014 Accepted 8 July 2014 Available online 5 August 2014 Keywords: Metal-particles Soot Nanostructures Biodiesel

a b s t r a c t In this study we show that the interaction of a solid metal in the form of wire in the post flame region formed using a biodiesel or fatty acid methyl ester (FAME) fuel (an oxygenated compound) can contribute significantly to the oxidation of the probe’s surface resulting in the deposition of metallic nanoparticles and carbon particulates with complex structural morphologies. The FAME used for forming the flame was canola methyl ester (CME). The interaction of the solid support within a flame medium formed using CME resulted in the formation of a distinct material deposition layer covering the surface of the probe. The formed layer was found to consist of clusters composed of aggregates of primary particles with a nearly spherical shape. The aggregates are composed of primary particles of carbon and of metallic characteristics. Other unique features include carbon networks containing numerous encapsulated ultra-small metal particles (<2.0 nm in diameter), elongated carbon nanofibers, metallic nanorods, and carbon–metal composites. High resolution transmission electron microscopy analysis reveals that the metal nanoparticles have a high degree of crystallinity. It is observed that the time and flame height parameters of the probe–flame interaction are important factors for varying the morphological characteristics of the deposits. Residence times ranging from 40 s to 5 min established a strong correlation to deposit morphology. Energy dispersive X-ray (EDX) analysis of material samples on the formed layers reveals the presence of carbon, iron, nickel, chromium and oxygen. The introduction of a probe with similar characteristics in the post flame region formed with No. 2 diesel fuel and air resulted in a thicker material layer covering the surface of the probe. Electron microscopy and EDX analysis showed that the deposits are composed mostly of carbon clusters and no metal content or other complex form of carbon morphology were detected. Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction During the last decade research efforts in the area of biofuels have been focused on synthesis processes, type of catalyst, and performance of these alternative fuels [1–8]. Numerous studies on biodiesels or fatty acid methyl esters (FAMEs) have been conducted on the thermo [2,9,10], catalytic pyrolysis [11–13], and heat of combustion. These fuels are seen either in blends with No. 2 diesel fuel or as ‘‘neat’’ biofuel (B100 or pure biofuel). Biodiesels are essentially a multicomponent mixture of long chain fatty acids and mono-alkyl esters [14]. Typical plants that have been utilized for FAME production include soy, canola, jatropha, rapeseed, and palm, among others. Biodiesels are oxygenated compounds and are known to have various advantages and disadvantages compared to traditional fuels. Among the advantages is the decrease ⇑ Corresponding author. E-mail address: [email protected] (W. Merchan-Merchan).

of combustion by-product emissions including particulate matter as well as total hydrocarbons and carbon monoxide during a combustion process. A negative of biodiesel, however, is the increase of nitrogen oxides (NOx) during the combustion process. Recently, Merchan-Merchan et al. [15] studied the sooting behaviors of various FAMEs including B100 CME (canola methyl ester), B100 SME (soybean methyl ester), a blend of 50% SME/50% animal fats, and No. 2 diesel fuels. In that study, utilizing the thermophoretic sampling technique and transmission electron microscopy (TEM), it was shown that particle size of the soot generated in all tested biodiesel–air flames is much smaller than that produced in the diesel– air flame. The smaller the size of the particles, the greater the potential for harm. Due to their size and light weight soot particles can be suspended in the air for weeks and travel long distances before settling. Recent research has also been conducted on the reactivity of liquid bio-alcohols and biodiesels with metal surfaces and has shown that biodiesels are more corrosive than diesel. Several studies have recently reported the corrosive effect on metal

http://dx.doi.org/10.1016/j.combustflame.2014.07.007 0010-2180/Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

W. Merchan-Merchan, H.O. Tenadooah Ware / Combustion and Flame 162 (2015) 216–225

surfaces exposed to biodiesel. Biodiesels and bio-alcohols erode and corrode the metal at the fuel film/metal interface and increase the metal content in the fuel [16–22]. It has been reported in recent studies that biodiesel fuels become even more reactive with the surface of a metal with a very slight increase of temperature (80 °C) [16]. What can be gained from these and other such biodiesel/metal corrosion studies is that surface metals exposed to biodiesels and bio-alcohols corrode much more rapidly than when the metal surfaces are exposed to petroleum-based fuels. Despite all of these research studies, very few studies have been devoted to occurrences within the biodiesel flame itself. In this work we study the reactivity of a CME–air diffusion flame by introducing a 0.6 mm-diameter nichrome probe in the post-flame yellow luminous zone of the flame. The tests were conducted using two different fuels, CME (C19H36O2) and Diesel (C16H34). A layer of carbon material was formed on the surface of the nichrome probe after introduction into the post-flame region for both types of produced flames. Electron microscopy and electron diffraction X-ray (EDX) analysis of the formed layer in the CME–air flame show unique physical and chemical characteristics compared to those present in the layer formed using diesel–air.

2. Experimental setup and procedures The experimental setup employed in this study consists of a wick burner, unislider assemblies, a Pyrex cylinder and a probe stabilizer assembly (Fig. 1). A cotton wick absorbs and transports the liquid fuel from the fuel chamber located at the bottom part of the burner to the other end of the wick where the fuel vaporizes and combusts with air to form a flame. The fuel and oxidizer are not pre-mixed prior to entering the burn zone. Through this method a laminar diffusion flame was formed using CME biodiesel and diesel fuel. The burner nozzle, with a diameter of 4.6 mm, forms a

stable flame inside the Pyrex cylinder, which acts as a draft deflector to stabilize the flame. Both ends of the Pyrex chamber are open. A distance of a few mm was kept between the bottom of the Pyrex cylinder and the base of the burner holder platform to provide a stream of continuous air to the burn zone (Fig. 1). Two burners of identical characteristics were employed in this study; one for creating the biofuel–air flame and the other for the diesel–air flame. The probe assembly is mounted on a movable platform, which allows for precise control of the probe’s axial height within the flame. The physical and chemical characteristics of the solid probe are a 0.6 mm-diameter wire consisting of 73%Ni + 17%Cr + 10%Fe (nichrome). The burner is mounted on a two-dimensional unislider assembly, which is driven by an 8300 series stepping motor controller (Model VXM-2, Velmex, Inc.). The vertical and horizontal unisliders allow the burner to move up and down as well as forward and backward as necessary. Fuel employed for this study was B100 canola methyl ester (CME) biodiesel. CME (C19H36O2) fuel is composed of the following compounds with the approximate different weight percentages: Methyl Palmitate (C17H34O2) (5%), Methyl Stearate (C19H38O2) (3%), Methyl Oleate (C19H36O2) (62%), Methyl Linoleate (C19H34O2) (20%), and Methyl Linolenate (C19H32O2) (10%) [23]. Similar experiments were also conducted using a fossil fuel for comparison. For both types of flames, the fuel flow rate is fixed, as the fuel is absorbed through a wick. The fossil fuel used in this study is an ultra-low sulfur diesel (ULSD) No. 2 obtained locally. Like CME and other biodiesels, diesel fuel is a mixture of several organic compounds mostly composed of saturated hydrocarbons, aromatics, and it can contain small amounts of dye and other additives. A typical low-sulfur No. 2 diesel fuel contains mostly saturated hydrocarbon components (75.3% by volume) and aromatic components (24.7%). The flowrate of the fuels are approximately 9.41 and 16.48 ml/h for CME and diesel, respectively. Corresponding to a fuel velocity for CME and diesel of 0.016 and 0.027 cm/s,

Exhaust

Hood

217

Pyrex Chamber

Flame

Burner Probe Fuel

Probe Stabilizer

v v 2-D Positioner

Fig. 1. Schematic of the experimental setup.

Computer

218

W. Merchan-Merchan, H.O. Tenadooah Ware / Combustion and Flame 162 (2015) 216–225

respectively. The flame is under atmospheric conditions and the oxygen content in the oxidizer stream to form the flame remains constant at 21 percent. During the sampling process the burner was kept at a fixed position. The surface scans of the as-grown layer on the probe were performed using scanning electron microscopy (SEM) (Zeiss Neon-40 EsB FIB-SEM). Detailed characteristics of the material forming the layers were obtained using transmission electron microscopy (JEOL TEM-3010). The specimens for TEM analysis were prepared by ultrasonic dispersion of the deposits collected from a specific flame position and immersed in methanol. The solution was then sonicated for a few minutes. A single drop of the solution was placed onto a perforated carbon TEM grid and the solvent was allowed to evaporate. Energy dispersive X-ray spectroscopy (EDX) was employed to characterize the chemical composition of the material forming the layer on the surface of the probe. The probe–flame interaction times varied from 50 s to 5 min and insertion into the post flame regions occurred at approximately middle-flame-height and upper-flame-height. The upper flame height is near the tip of the flame.

Figure 2(b1) represents typical material characteristics present on the surface layer after the probe was inserted in the flame medium for 120 s. Higher resolution SEM analysis (Fig. 2b2) shows typical aggregates formed of nearly spherical shaped particles but also reveals the presence of elongated or ‘‘strand-like’’ connections between the aggregates. The ‘‘strands’’ seen here have very small diameters, less than 30 nm across and are of hundreds of nanometers in length. The characteristics of the material layer formed on the surface of the probe with a probe–flame interaction time of 240 s are represented in Fig. 2(c). Similar to the previous case, ‘‘strand-like’’ structures are present and are accompanied by aggregates formed of nearly spherical structures, (Fig. 2c1 and c2). TEM, HR-TEM and EDX analysis were used to obtain the structural and chemical details of the material forming the layer. For TEM analysis a solution was formed by releasing deposits from the surface of the nichrome probe in a methanol volume. A drop of the solution was placed onto a perforated carbon TEM grid and dried at room temperature. Figure 3a–c represents low- and HR-TEM images along with EDX spectra of the material layer formed on the surface of the nichrome probe after insertion into the CME–air flame at Z = 12 mm for a probe–flame exposure of 50, 120 and 240 s, respectively. Imaging analysis conducted on samples of the material formed at the lowest probe–flame interaction time, 50 s, shows the layer to be composed of typical soot agglomerates (Fig. 3a1). The agglomerates are composed of tens of primary particles that have a nearly spherical shape, Fig. 3a1. Applying HR-TEM to a selected primary particle reveals that it has a shell-core structure. The structure is composed of closed stacked long-range well defined carbon layers that are concentrically arranged along the outermost periphery forming the particle (Fig. 3a2–a3). It is also evident that the particle’s core is composed of short and less organized carbon layers. The collected EDX spectrum from a selected area on the particles aggregate shown in Fig. 3a1 using

3. Results and discussion 3.1. CME–air flame The low and high resolution scanning electron microscopy (HRSEM) images in Fig. 2 represent morphological characteristics collected on the surface of the as-grown layer formed on the surface of the nichrome probe after insertion in the CME–air flame at the height of Z = 12 mm at different probe–flame interaction times. Aggregates formed of nearly spherical shaped primary particulates appear to be forming the material layer on the surface of the nichrome probe after insertion in the flame for 50 s as revealed by the high resolution SEM (Fig. 2a2).

(a)

(c)

(b) (a1)

200 nm

100 nm

200 nm (a2)

100 nm

(c2)

(b2)

30 nm 50

(c1)

(b1)

100 nm 120

240

Time (sec)

Fig. 2. Characteristics of the material deposits covering the surface of the nichrome probe formed after the insertion of the probe in a CME–air flame volume (Z = 12 mm) at different probe–flame exposure times. Z represents the distance from the edge of the burner nozzle to the surface of the probe.

219

W. Merchan-Merchan, H.O. Tenadooah Ware / Combustion and Flame 162 (2015) 216–225

(b)

(a)

(c) (b1)

(a1)

(c1)

50 nm

100 nm

100 nm

(b2)

(a2)

(c2) 5 nm 5 nm

5 nm (a3)

(c3)

(b4)

(b3) 100 nm 15000

C

C o u n t s

Cu O

Cr C O Fe

Cu

0,0

2.0

4.0

6.0

8.0

10.0

keV

Cu

(c4)

(b5)

(a4) C o u n t s

(c3) 7000

20 nm

0,0

2.0

Cu

Ni

Cu

4.0

6.0

8.0

10.0

keV

Time (s) Fig. 3. Low and HR-TEM images revealing detailed characteristics of the deposits collected from the surface of the nichrome probe after insertion in a CME–air flame at the height of Z = 12 mm at different probe–flame interaction times.

TEM (Fig. 3a4) contains peaks of carbon, oxygen, and copper. The copper signals are attributed to the nature of the TEM grid (copper substrate). A fraction of the carbon peak in the EDX spectrum should account for the C film present in the TEM grid used for the analysis. TEM analysis conducted on the samples formed by the introduction of the probe in the post flame region (Z = 12 and 18 mm) for a residence time of 2 and 4 min reveals that the deposits are composed mainly of clusters consisting of soot aggregates. These aggregates are characterized by a high degree of agglomeration. However, within the typical soot aggregates other structures containing elongated morphology (structures with link-like shape), highly organized carbon nanomaterials, and microstructure variations of the soot primary particles are evident (Fig. 3b and c).

Applying HR-TEM on a selected particle of the aggregate shows that the soot experienced essential modification of soot morphology (Fig. 3b2). The carbon layers are less defined and possess various degrees of curvature (Fig. 3b2 and b3). It is also evident that the sample is composed of nanoforms that closely resemble carbon nanofibers (CNFs) and carbon ropes composed of single walled nanotubes (SWNTs) such as the structures shown in Fig. 3b4 and b5, respectively. The CNF in Fig. 3b4 is surrounded by soot aggregates. It is worth noting that very carefully designed experiments have been conducted in order to form SWNTs in flames as it requires the introduction of metal particles at a certain phase condition and size [24–28]. In the flame method the catalytic precursors are generally introduced into the flame system in the gas-

220

W. Merchan-Merchan, H.O. Tenadooah Ware / Combustion and Flame 162 (2015) 216–225

phase and nucleate and condense to solidify into ultra-small size spherical metallic nanoparticles [29]. Similar to synthesis in other processes, in this study the SWNT are formed in bundles and are entangled with each other as shown in HR-TEM images in Fig. 3b5. Several SWNTs can be observed to be forming the rope. An average diameter of the SWNTs of 4.2 nm was measured. TEM analysis conducted on samples of the material layer formed on the surface of the probe inserted at a probe–flame interaction time of 4 min clearly shows the presence of ‘‘strand-like’’ structures surrounded by typical soot aggregates (Fig. 3c1). The physical characteristics of the ‘‘strand-like’’ structures observed in TEM correspond to those observed in the SEM analysis. High resolution TEM of selected areas of the ‘‘strand-like’’ structures shows them to be composed of individual coalesced particles with long ranging carbon layers and hollow core (Fig. 3c2 and c3). Another interesting characteristic of this sample is the presence of the ultra-small particles (darker contrast of a few nm in diameter) that are attached or embedded to/in the surface of the fiber as pointed out by the arrow in Fig. 3c2. The point EDX spectrum collected on the small particle of darker contrast contains C, O, Cr, Fe, Co, and Ni peaks. The Cu peak in the spectrum is from the TEM grid walls. Song et al. [30] exposed soot particulate samples formed in a 6-cylinder turbodiesel engine running with a FAME and alternative (diesel) fuels to different burning modes to study the mechanism by which biodiesel soot enhances oxidation. In that study it was shown that carbon particulates from neat biodiesel (B100) are more reactive during oxidation than soot formed with a diesel fuel. The soot produced from the FAME undergoes a unique oxidation process leading to the transformation of the solid circular carbon particles into a hollow core capsule-type structure and eventually to graphene ribbon nanoforms. The probe–flame interaction was also studied by repositioning the nichrome probe in the post-flame region (Z = 18 mm) of the CME–air flame. The temperature of the flame increases as we move

(a)

in the direction of the flame’s tip. At 18 mm the temperature of the CME–air flame is approximately 840 °C [15]. Figure 4 presents SEM images of the typical morphological characteristics collected on the surface of the as-grown material layer formed on the surface of the probes at different probe–flame interaction times. The SEM image (Fig. 4a1) reveals that ‘‘strand-like’’ fibers are already present within the material layer at this low probe–flame interaction time (50 s). The application of HR-TEM imaging on a selected area (in Fig. 4a1) clearly shows the presence of elongated structures (as pointed out by the black arrows) surrounded by large aggregates composed of primary particles with a high degree of agglomeration, Fig. 4a2. The nichrome probe was further given a probe–flame interaction time of 120 s. SEM scanning of the surface of the formed material layer reveals that the ‘‘strand-like’’ fibers are superior defined; they have long lengths and narrow diameters. This type of structure is clearly distinguishable. A close up view of a selected area in Fig. 4b1 shows that the ‘‘strand-like’’ fibers exhibit a high degree of networked morphology or ‘‘web-like’’ structure, Fig. 4b2. The further increase of the nichrome probe–flame interaction time (5 min) resulted in ‘‘strand-like’’ fibers (Fig. 4c) with much longer lengths than the lengths of the structures formed at the previous probe–flame interaction times (Fig. 4c2). Another interesting characteristic is the presence of larger single particles and aggregates (background in Fig. 4c2) that are composed of a low degree of agglomeration of spherical particles. Similarly, information on the internal morphology of the deposits formed on the surface of the nichrome probe inserted in the post flame region at Z = 18 mm was obtained by employing TEM and HR-TEM. Figure 5a1 represents a lower resolution TEM image of the deposits (for a probe–flame exposure of 50 s) showing an aggregate composed of primary particles. The aggregate is composed of individual particles that have a lighter and darker contrast under the electron beam. Applying HR-TEM to some of the darker contrast particles reveals that the particles are metallic and have a high degree of crystallinity (Fig. 5a2 and a3). It is also clear that

(c)

(b)

(a1)

100 nm

100 nm

100 nm

(a2)

100 nm

(c2)

(b2)

20 nm 50

(c1)

(b1)

100 nm 120

300

Time (s)

Fig. 4. Characteristics of material deposits covering the surface of a nichrome probe formed after the insertion of a probe in a CME–air flame at different times. The probes were inserted at the flame height of Z = 18 mm from the edge of the burner nozzle.

221

W. Merchan-Merchan, H.O. Tenadooah Ware / Combustion and Flame 162 (2015) 216–225

(b)

(a)

(c) (c1)

Carbon

(b1)

(a1)

Metals

50 nm

20 nm

100 nm

(b2)

(a2)

Nanometals

Metal

Metal Carbon

(c2) 2 nm Metal

Carbon

(c3)

(b3)

5 nm

Metal

(a3)

(c4)

(a4)

Carbon

(c5) 10 nm

(a5)

6600 C o u C O n t Cu s

Metal

0,0

~0.39 nm 20 nm 50

20 nm

(b5)

(b4) 120

Metal 50 nm

Cu 14000

Cr Fe Cu Ni

2.0 4.0 6.0 8.0 keV

(c6)

C o u n t s 0,0

C

O Cu

Cu Ni Cu Cr Fe

2.0 4.0 6.0 8.0 keV

(c7) 300

Time (s)

Fig. 5. Low and HR-TEM images revealing detailed characteristics of deposits collected from the surface of the nichrome metal probe after inserted in a CME–air flame at the height of Z = 18 mm at different probe–flame interaction times.

some of the metallic particles are completely imbedded in carbon. The carbon embedded metallic particles in Fig. 5a2 are nearly spherical with a diameter as small as eight nanometers. Similar to the soot particles the metallic particles are nearly spherical forming aggregates. The presence of networked structures is clearly visible within the deposits as highlighted by the arrow in Fig. 5a5. HR-TEM on the carbon material shows that the carbon layers forming a particle possess various degrees of curvature (Fig. 5a4). TEM analysis on samples of the material layer formed on the surface of the probe by increasing the probe–flame exposure time to 120 s reveals an increased number of networked structures (Fig. 5b1). It appears that these networked structures are composed of link-like chain particles that are fused together to form the ‘‘strand-like’’ structures (Fig. 5b1) and are quite transparent under the electron beam. Similar to the previous case, the networked structures are accompanied by spherical particles that

have a darker contrast under the microscope (Fig. 5b3). HR-TEM imaging of a typical structure (circled area in Fig. 5b3) at the atomic level shows that the structures have well defined metallic lattice fringes, Fig. 5b4. HR-TEM reveals that some of the lighter contrast particles forming the aggregates are composed of nonclosed stacked planar carbon sheets as pointed out by the dotted arrow in Fig. 5b3 and b5. In the study reported by Song et al. [30] the structural characteristics of B100 biodiesel soot were also studied through TEM analysis after the soot was exposed to 500 °C in air and reached 40% and 75% burnoff. TEM analysis showed that B100 soot undergoes a unique oxidation process leading to capsule-type oxidation and eventual formation of graphene ribbon structures. The individual primary particles exposed to higher burnoff times become hollow inside to form elongated structures resembling short multiwalled carbon nanotubes. In their work Song et al. concluded that increased surface oxygen functionality in the B100 soot could

222

W. Merchan-Merchan, H.O. Tenadooah Ware / Combustion and Flame 162 (2015) 216–225

offer the means for more rapid oxidation and cause radical structural transformation during the oxidation process [30]. TEM micrographs taken from samples of deposits on the surface of the probe for the 5 min residence time again reveal material clusters composed of agglomerates formed of primary particles surrounded by more ‘‘strand-like’’ fibers with a high degree of networked morphology (Fig. 5c). In Fig. 5c1 it is observed that the ‘‘strand-like’’ fibers are composed of materials that have a light and darker contrast. Figure 5c2 represents a high magnification from a selected area of the ‘‘strand-like’’ fiber in Fig. 5c1. The ‘‘strand-like’’ fiber contains a high number of metal nanoparticles. Surprisingly, the metal particles have an ultra-small diameter (few nanometers) and are completely embedded within the lighter contrast material (carbon), thus forming a networked carbon–metal composite structure. The upper and lower inserts in Fig. 5c2 represent HR-TEM of a selected area of the network composite. The ultra-small particles are embedded in carbon (lower insert in Fig. 5c2) and have lattice fringes that are very distinguishable (upper insert in Fig. 5c2). Another interesting type of nanostructure observed at this flame position and time parameter are metallic elongated structures (rodlike shape) surrounded by aggregates composed of nearly spherical primary particles (Fig. 5c3). The rods have less than 20 nm width and are less than 50 nm in length. HR-TEM analysis of the rod structures reveals that they have well defined lattice fringes (Fig. 4c4). HR-TEM on the lighter contrast material forming the aggregates shows that they are composed of curved and planar stacked carbon sheets with d-spacing of 0.35 nm which is typical of graphite (Fig. 5c5). The EDX spectrum collected on the networked structures containing the ultra-small size diameter metallic particles exhibits peaks of C, O, Cu, Cr, Fe, and of Ni (Fig. 5c7). The C and Cu peaks are significant compared to the other metal peaks in the spectrum. The EDX spectrum collected on the elongated metal nanorods (circled area in Fig. 5c6) contains C, O, Cu, Cr, Fe, and Ni peaks (Fig. 5c6). However, the O and Cr peaks are much more pronounced compared to the other peak elements in the spectrum. The C and Cu peaks are partially related to the grid’s carbon film and walls, respectively. We hypothesize that the complex nanomaterials observed in this study may be the result of a combination of oxidation and possible corrosion effects on the surface of the metal probe while it is positioned in the CME–air flame volume. The oxygen present in the large ester compounds that compose the biodiesel fuel can be a plausible explanation for the oxidation of the probe and the inception of complex structures/composites that can be observed regularly accompanying the soot particulates formed in the biodiesel– air flame. Saito et al. reported the physical and chemical soot characteristics in nine different hydrocarbon (non-oxygenated compounds) fueled diffusion co-flow flames including: methane; ethane; acetylene; propane; allene; 1-butene; 1,3 butadiene; and benzene with air as the oxidizer [31]. In that work most of the carbon deposits where composed of clusters of soot primary particles. XPS analysis showed that the clusters of the soot primary particles deposited on the solid support had both adsorbed oxygen and oxygen chemically bound to carbon. The alkyl esters produced by the transesterification of vegetable oils and animals fats result in a mono-alkyl ester that contains long-chain fatty acids of 16–18 carbons that are mostly aliphatic and monocarboxylic and tend to have very corrosive characteristics at temperatures even as low as 60 °C above room temperature as shown by Fazal et al. [16]. The presence of structures that resemble SWNTs requires an additional explanation. The esters in biodiesel have been observed as chemically corrosive to metal surfaces of iron [16]. The chemical corrosion rate is small at room temperature, but the rate increases as temperature increases [16]. This effect has not been tested in high temperatures, not to mention in very high temperatures as

those present in a flame medium. The corrosion effect could be the reason distinct metallic particles of suitable size for SWNT growth appear. The temperature within the flame (Z = 12 mm) is 770 °C, which represents a significantly higher temperature for corrosion studies of biodiesels than those employed by Fazal et al. and others [16]. In recent years several research groups have devoted great efforts to understanding the underlying mechanisms of the flame synthesis of multiwalled carbon nanotubes and related nanostructures [32–36]. In those works flame parameters, type of catalyst, fuel type (carbon source), and probe–flame residence time (among others) were employed. In this work we show that carbon–metal structures of unique morphology are present among the soot particulates produced in a B100 CME–air flame. The biodiesel produced soot is accompanied by nearly spherical and elongated metallic particles, carbon networks and carbon–metal composites comprised of surprisingly ultra-small metallic particles embedded in the soot (a few nanometers in diameters). Much further research is necessary to obtain a more detailed mechanism of the formation of these carbon–metal structures in the biodiesel–air flame. FAMEs have higher cetane numbers, approximately 10% oxygen in the chemical molecular formula, and lack aromatics [37]. These FAME fuels have many advantages and disadvantages in terms of exhaust emissions and heating values, among others. CO emissions have been shown to be up to 18% lower for neat FAMEs compared to neat diesel, and unburned hydrocarbons over 40% [37]. The particulate matter from neat biodiesel is also shown to be significantly reduced in size, number, and volume [38,15]. That is, particle diameters can be reduced nearly 25%, number by 38%, and volume by 82%. These reductions are due in large part to the oxidation rate of biofuels which can be as much as six times that of diesel. NOx emissions increase for biofuels however, as much as 12% because of the oxygen content found in these fuels [37], and carbonyl emissions also increase proportionally to the concentration of biodiesel in a blend [39]. In addition, the fatty acids which are present in biodiesels have been studied in order to understand the effects of various biodiesel compositions on exhaust emissions [40]. It was found that the acid density, cetane number, and iodine number (number of double bonds) are all correlated and have various consequences for biodiesel emissions. NOx emissions were shown to increase along with density, while particulate matter decreased. NOx was also shown to increase as the length of the fatty acid chain decreased, or as the iodine number increased, or as the cetane number decreased. Furthermore, it has been reported in recent studies that the same components present in the biofuel composition that provides certain advantages over traditional fuels can also yield disadvantages such as the enhancement of the corrosion effect on the metallic surfaces exposed to the biodiesel fuels. It has been reported in recent studies [16] that esters present in CME and other FAMEs are very reactive with some metallic surfaces due to the physiochemical properties of biodiesels. The same group has also shown that the biodiesel fuels become even more reactive with the surface of a metal with a very slight increase of temperature in the medium. This could explain the high metallic peaks seen in both ‘‘soot’’ and ‘‘composite’’ samples of our experiments. That is, in our experiments the high density of complex forms of carbon, metal and metal–carbon composites present within the deposits can be attributed to the reactivity of the surface of the metal wire with the CME–air flame. Corrosion experiments with biodiesels (both Static Immersion Tests and Dynamic Immersion Tests) have shown esters and free fatty acids break down metals such as copper, aluminum, and other engine hardware at higher rates at low temperatures (50– 80 °C) [16,18,21,22]. Our study represents a dynamic immersion test accomplished at high temperatures.

223

W. Merchan-Merchan, H.O. Tenadooah Ware / Combustion and Flame 162 (2015) 216–225

3.2. Diesel–air flame

3.3. Probe surface analysis CME and diesel

To support the above hypothesis additional experiments were conducted. The experiments were repeated by introducing the probe in a flame medium formed with a No. 2 diesel. Similar to the biodiesel experiments, probes were placed within the Diesel– air flame at two heights: mid-flame (12.7 mm) and near-tip height (19.0 mm) and at different probe–flame interaction times (50 and 300 s). Figure 6 represents typical characteristics of structures deposited on the surface of a probe inserted in the diesel–air flame at Z = 19 mm at a probe–flame interaction time of 300 s. The low resolution TEM image (Fig. 6a) is a typical aggregate composed of tens of soot primary particles. The aggregate is formed of particles of light- and dark-contrast as pointed out by the dotted and solid arrows in Fig. 6a, respectively. HR-TEM analysis of randomly selected soot of the formed aggregate shows that the internal structure of the soot is composed of disturbed graphene layers stacked together and forming a particle due to the strong bonds and curvature (Fig. 6b). Despite great efforts no signs of metallic particles were observed using HR-TEM or the presence of bright spots formed near the surface of the particles as would typically appear when the electron beam interacts with a metallic particle. X-ray diffraction analysis was also conducted on the aggregates (Fig. 6c). The EDX spectrum collected from the aggregate contains C, O and Cu peaks. The signal of the Cu in the spectrum is from the walls of the TEM grid. A fraction of the carbon should also be part of the C film used for the analysis. The oxygen content within the soot is consistent with the study of Song et al. [30]. Elements forming the probe such as Ni, Cr and Fe are not present in the aggregate. Probes positioned at the axial diesel–air flame height of Z = 12.7 mm were also tested. For both probe–flame exposure times, the deposits typically appeared to be composed of primary soot particle aggregates (not shown here). The study reported by Ishiguro and coworkers shows that the soot produced from a diesel fueled CI engine is composed of a double structure [41]. The outer shell consists mostly of graphitic material. The inner core consists of several ‘‘fine’’ particles that are approximately 3–4 nm in diameter. The nucleus of the fine particles was not well arranged (having a ‘‘turbostratic’’ structure). It is reported that the morphology of the soot characteristics (outer and inner shells) is common to particles under all operating conditions tested. A close comparison/inspection of the internal structure of the soot produced in the biodiesel–air flame reveals that the soot produced in the CI engine is quite different. Furthermore, the soot produced in the present study (diesel–air flame) does not contain the double structure and cores composed of ‘‘fine’’ particles as observed in the work of Ishiguro et al. [41].

Additionally, the results were also supported by studying the surface characteristics of the nichrome probe as commercially obtained and after introduction into a diesel–air and CME–air flame. The probes where inserted at different probe–flame interaction times and at approximately middle of the flame height. After the probes were inserted in the flame medium the deposits were removed from the surface by immersing them in a methanol solvent and through a sonication process of a few minutes. SEM imaging and EDX analysis were conducted in selected areas on the surface of a nichrome probe: (a) as commercially obtained, (b) after introduction in a flame medium formed using No. 2 diesel and (c) after the probe was introduced in a flame volume formed with CME as the fuel (Fig. 7a–c). The low resolution SEM image (Fig. 7a1) collected on the surface of the probe ‘‘as commercially obtained’’ reveals that the surface of the probe is composed of grain and slips. Some of the boundary lines form ‘‘islands’’ (Fig. 7a2). Electron diffraction Xray collected on the surface of the ‘‘as commercially obtained’’ probe shows the peaks present in the spectrum are Ni, Cr, and Fe which are the elements present in the alloy (EDX spectrum in Fig. 7a). With nickel being the primary element of the alloy, nickel has the higher peaks within the spectrum. It is also important to note that the tested probes are composed of 73%Ni, 17%Cr and 10%Fe. In a nickel alloy the Ni element promotes corrosion resistance. A comparison of Fig. 7b and c clearly reveals the presence of a surprising variation in the surface morphology of the nichrome probes after inserted in the flame volume formed using No. 2 diesel and CME, respectively. The probe inserted in the CME–air flame (30 s residence time) has a thin layer composed of unique elongated structures strutting from its surface. Some of the strutting structures appear to be short and thick, 3D packed, and have cross sectional areas of micron-size (Fig. 7c1). SEM images in Fig. 7b2 and c2 depict the morphological characteristics present on the surface of the probes inserted in a diesel–air and CME–air flame with a probe–flame exposure time of 120 s. It is evident that the most remarkable variation of the surface physical characteristics occurred as the probe–flame exposure time was increased to 120 s in CME–air flame. That is, the surface of the probe is covered with the thin oxide layer formed of unique densely packed micronsize structures that are slender, prismatic and multi-faced (Fig. 7c2). Structures with similar characteristics in a combustion synthesis method have been formed only when using oxygenenriched air that usually results in a high temperature flame medium [42]. The EDX spectra in Fig. 7(b) and (c) were collected on the surface of the probe after insertion in the diesel–air and CME–air 18000

C o u n t s

0,0

5 nm

100 nm

(a)

(b)

C

O

Cu

Cu

Cu 2.0

4.0

6.0

(c)

8.0

10.0

keV

Fig. 6. Low and HR-TEM images of deposits collected from the surface of a nichrome probe after inserted in a flame volume formed using No. 2 diesel and air as the oxidizer. From upper flame region (Z = 19 mm) at 300 s probe–flame exposure.

224

W. Merchan-Merchan, H.O. Tenadooah Ware / Combustion and Flame 162 (2015) 216–225

Time (sec)

Diesel/air Flame

As Commercially Obtained

CME/air Flame (b2)

(a2)

100nm C o

u n t s

400

120

Ni

0,0

2.0

4.0

Fe

Ni

6.0

8.0

100nm

C o u n t s

Ni

Cr

10.0

keV

300

Ni

O Cr Fe

0,0

2.0

4.0 6.0

Ni

C o u n t s

Ni Ni

8.0 10.0

O Ni

Cr Fe

0,0

2.0

4.0

300nm

(b)

Ni

6.0 8.0 10.0

keV

keV

(b1)

30

(a)

20nm 160

(a1)

200 nm

(c2)

(c1)

200nm

(c)

Fig. 7. Representative SEM images and EDX spectra collected on the surface of nichrome probes: (a1 and a2) ‘‘as commercially obtained’’ probe: (b1 and b2) after the nichrome probe was inserted in the post region of a flame volume formed using diesel fuel; and (c1 and c2) after the nichrome probe was inserted in the post region of a flame volume formed using CME. Air was used in both cases as the oxidizer. SEM scans on the surface of the tested nichrome probe were conducted on the surface of probes inserted at different probe–flame exposure times and at the middle of the flame height.

flame, respectively. The oxygen peak collected from the surface of the probe inserted in the CME–air flame is much higher than that collected on the surface of the probe inserted in the diesel–air flame (EDX spectrum in Fig. 7b and c). The work of Fazal et al. [20] showed that an oxide layer is formed on surfaces of Cu and Al after exposure to biodiesel at 80 °C for 1200 h. In the same study it was reported that by exposing a palm biodiesel (with major components of palminate: 16:0; 44.272% and Methyl Oleate: 18:1; 35.934% esters) to the temperature of 80 °C it resulted in the formation of new acids [20]. Upon heating of the biodiesel without exposure to the metals, the newly produced acids included decanedioic acid, nanonoic acid, hydroxypentadecanoic acid, 15hydroxypentadecanoic acid, among others. In addition to these acids, different types of other short chain esters, aldehydes, ketones were generated under both conditions. The oxygenated compounds present within biodiesels can be easily regenerated into new acids (fatty acids) as they are exposed to high

temperature media. These newly generated fatty acids can significantly increase the corrosiveness of the biodiesel as they are exposed to the surface of the metal probe. The flame temperature for the CME–air at Z = 9 mm (near region of the fuel nozzle) and Z = 21 mm (region near flame tip) was measured to be approximately 730 °C and 910 °C, respectively [15]. For the diesel–air flame temperature measurements the surface of the thermocouple (rapid insertion technique) resulted in a heavier accumulation of soot on its surface which could result in inaccurate flame temperature measurements [43,15]. However, Glaude et al. used a thermochemical approach to evaluate the adiabatic flame temperature of various FAMEs and petroleum derived fuels including diesel [44]. In that study it was shown that biodiesel fuels lead to adiabatic flame temperatures that are slightly lower than those formed in diesel fuel. Therefore, suggesting that the morphological characteristics of the carbon and carbon–metal particulates present on the surface of the probe were enhanced due

W. Merchan-Merchan, H.O. Tenadooah Ware / Combustion and Flame 162 (2015) 216–225

to the corrosiveness of the oxygenated fatty acids fuel forming the flame rather than a temperature effect alone. 4. Conclusions Morphological characteristics were studied on material layers formed on the surface of a nichrome probe when the metal probe was introduced in the flame volume formed using a traditional fuel and a FAME. The flames were formed using CME (oxygenated fuel) and ultra-low sulfur diesel (ULSD) (No. 2) both using air as the oxidizer in a wick-based burner. The characteristics of the formed deposits were studied by varying the residence time of the probe inside the flame and by changing the position of the probe within the flame volume. SEM and TEM studies of the material layer formed when using the oxygenated fuel revealed the presence of elongated carbon structures, metal, and carbon–metal composites besides the typical soot clusters. We hypothesize that the variety of morphological effects of the carbon–metal nanoforms present within the soot formed in the biodiesel flame could be attributed to a combination of processes including: (i) the fact that the soot formed in the B100 CME is produced with an oxygenated fuel that can yield carbon particulates with a unique oxygen functionality property, thus allowing for a faster oxidation resulting in severe alteration of the particulates. SEM and TEM imaging of the formed products strongly support this effect as the time of the probe– flame interaction is increased. (ii) Given that biodiesels are oxygenated compounds and given the fact that their major components (fatty acids) can be regenerated into new acids as they are exposed to high temperature media and metal surfaces, these characteristics can significantly increase the corrosiveness of the biodiesel and attack the surface of the probe (metal dissolution at the surface). The presence of composites formed of carbon with encapsulated ultra-small size metallic particles supports this effect. SEM analysis on the surface of the probe after the black material layer was removed, using a sonication process, shows the presence of a thin metal oxide layer on its surface after it is introduced in the CME–air flame. The thin oxide layer contains close packed micronsized 3D structures that are slender, prismatic and multi-faced strutting from its surface. Structures with similar shapes are commonly formed in oxygen-enriched air flames. Similar experiments were conducted with the No. 2 diesel–air flame and the layer of material deposited on the surface of the probe did not contain complex structures as present in the deposits formed using the B100 CME–air flame. The deposits were composed of typical carbon clusters formed of carbon aggregates with a high degree of agglomeration. Furthermore, the thin oxide layer formed on the surface of nichrome probe was not present when the probe was introduced in the flame formed with the No. 2 diesel fuel. The absence of the thin oxide layer is most likely due to the high reactivity (erosion/corrosion) that the FAME fuel gases have with a metal surface. Acknowledgments The support of this work by the National Science Foundation through the Research Grant; CBET-1067395 is gratefully acknowledged. The authors would like to extend special thanks to Dr. Preston Larson from the Samuel Roberts Noble Electron Microscopy Laboratory at the University of Oklahoma for help with the ZEISS NEON high resolution SEM and helpful discussions. We would also like to thank Dr. Alan Nicholls and Dr. Ke-Bin Low from the University of Illinois at Chicago Research Resource Center for assistance in TEM studies, encouragement and helpful

225

discussions. Henry Tenadooah Ware would like to thank Ms. Sophia Morren (McNair Scholars Director) at OU for all of her valuable comments and suggestions. The authors are also thankful to Mr. Stephen McCollam for his very valuable comments and helpful discussions. References [1] T. Nguyen, L. Do, D.A. Sabatini, Fuel 89 (2010) 2285–2291. [2] P.D. Patil, S. Deng, Fuel 88 (2009) 1302–1306. [3] L.T. Thanh, K. Okitsu, Y. Sadanaga, N. Takenaka, Y. Maeda, H. Bandow, Bioresour. Technol. 101 (2010) 639–645. [4] E. Alptekin, M. Canakci, Fuel 88 (2009) 75–80. [5] E. DeOliveira, R.L. Quirino, P.A.Z. Suarez, A.G.S. Prado, Thermochim. Acta 405 (2006) 87–90. [6] Z. Habib, R. Parthasarathy, S. Gollahalli, Appl. Energy 87 (2010) 1701–1709. [7] F.E. López Suárez, A. Bueno-López, M. Illán-Gómez, B. Ura, J. Trawczynski, Catal. Today 176.1 (2011) 182–186. [8] N. Lamharess, C.N. Millet, L. Starck, E. Jeudy, J. Lavy, P. Da Costa, Catal. Today 176 (2011) 219–224. [9] P.A. Glaude, R. Fournet, R. Bounaceur, M. Moliere, Fuel Process. Technol. 91 (2010) 229–235. [10] G. Rodrigues De Souza, A. Moreira Dos Santos, S. Lucas Ferreira, K.C. Ribeiro Martins, D.L. Modolo, Appl. Therm. Eng. 29 (2009) 2562–2566. [11] G. Guan, K. Kusakabe, N. Sakurai, K. Moriyama, Fuel 88 (2009) 81–86. [12] J. Zhang, S. Chen, R. Yang, Y. Yan, Fuel 89 (2010) 2939–2944. [13] A. Demirbas, Prog. Energy Combust. Sci. 31 (2005) 466–487. [14] F. Qiyao, A. Jalali, A.M. Fincham, Y.L. Wang, T.T. Tsotsis, F.N. Egolfopoulos, Combust. Flame 159 (2012) 1876–1893. [15] W. Merchan-Merchan, S.G. Sanmiguel, S. McCollam, Fuel 102 (2012) 525–535. [16] M.A. Fazal, A.S.M.A. Haseeb, H.H. Masjuki Masjuki, Proc. Int. Conf. Adv. Sci., Eng. Inf. Technol. (2011) 505–510. [17] L. Xiaoyuan, P.M. Singh, Corros. Sci. 52 (2010) 2303–2315. [18] L. Diaz-Ballote, J. Lopez-Sansores, L. Maldona-Dolopez, L. Garfiasmesias, Electrochem. Commun. 11 (2009) 41–44. [19] G. Asta, S.J. Asadauskas, D. Brazˇinskiene, I. Liašcˇukiene, A. Stoncˇius, Dissolved metals as soot precursors – review of dissolution mechanisms in thin biofuel films, in: Proceedings of COST CM0901 1st Annual Meeting, Nancy, September 2010. [20] M.A. Fazal, A.S.M.A. Haseeb, H.H. Masjuki, Fuel Process. Technol. 91 (2010) 1308–1315. [21] S. Kaul, R. Saxena, A. Kumar, M. Negi, A. Bhatnagar, H. Goyal, A. Gupta, Fuel Process. Technol. 88 (2007) 303–307. [22] B. Singh, J. Korstad, Y.C. Sharma, Renew. Sustain. Energy Rev. 16 (2012) 3401– 3408. [23] W.F. Fassinou, Energy 45 (2012) 798–805. [24] W. Merchan-Merchan, A.V. Saveliev, W. Cuello Jimenez, Proc. Combust. Inst. 33 (2011) 1899–1908. [25] H. Richter, M. Treska, J.B. Howard, J.Z. Wen, S.B. Thomasson, A.A. Reading, P.M. Jardim, J.B. Vander Sande, J. Nanosci. Nanotechnol. 8 (2008) 6065–6074. [26] C.J. Unrau, R.L. Axelbaum, P. Biswas, P. Fraundorf, Proc. Combust. Inst. 31 (2007) 1865–1872. [27] M.J. Height, J.B. Howard, J.W. Tester, J.B. Vander Sande, Carbon 42 (2004) 2295–2307. [28] R.L. Vander, Wal. Combust. Flame 130 (2002) 37–47. [29] R.L. Vander Wal, L.J. Hall, Combust. Flame 130 (2002) 27–36. [30] J. Song, M. Alam, A. Boehman, U. Kim, Combust. Flame 146 (2006) 589–604. [31] K. Saito, A.S. Gordon, F.A. Williams, W.F. Stickle, Combust. Sci. Tech. 80 (1991) 103–119. [32] L. Yuan, K. Saito, C. Pan, F.A. Williams, A.S. Gordon, Chem. Phys. Lett. 340 (2001) 237–241. [33] R.L. Vander Wal, T.M. Tichich, V.E. Curtis, Chem. Phys. Lett. 323 (2000) 217– 223. [34] F. Xu, X. Liu, S.D. Tse, Carbon 44 (2006) 570–577. [35] S. Sen, I.K. Puri, Nanotechnology 15 (2004) 264–268. [36] W. Merchan-Merchan, A.V. Saveliev, L. Kennedy, W.C. Jimenez, Prog. Energy Comb. Sci. 36 (2010) 696–727. [37] M. Canakci, J.H. Van Gerpen, Trans. ASAE 64 (2003) 937–944. [38] J. Heejung, D.B. Kittelson, M.R. Zachariah, Environ. Sci. Technol. 40 (2006) 4949–4956. [39] S. Machado Correa, G. Arbilla, Atmos. Environ. 42 (2008) 769–775. [40] R.L. McCormick, M.S. Graboski, T.L. Alleman, A.M. Herring, Environ. Sci. Technol. 35 (2001) 1742–1747. [41] T. Ishiguro, Y. Takatori, K. Akihama, Combust. Flame 108 (1997) 231–234. [42] W. Merchan-Merchan, A.V. Saveliev, A. Taylor, Micron 40 (2009) 821–826. [43] K. Saito, F.A. Williams, A.S. Gordon, Combust. Sci. Technol. 47 (1986) 117–138. [44] P.-A. Glaude, R. Fourneta, R. Bounaceura, M. Molière, Fuel Process. Technol. 91 (2010) 229–235.