Comparison of Soot Formation, Evolution, and Oxidation Reactivity of

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A comparison of soot formation, evolution and oxidation reactivity of two biodiesel surrogates Zhan Gao, Lei Zhu, Chunpeng Liu, Ang Li, Zhuoyao He, Congfei Zhang, and Zhen Huang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00922 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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A comparison of soot formation, evolution and oxidation reactivity of two biodiesel surrogates Zhan Gao, Lei Zhu*, Chunpeng Liu, Ang Li, Zhuoyao He, Congfei Zhang, Zhen Huang* Key Lab. for Power machinery and Engineering of M. O. E., Shanghai Jiao Tong University, Shanghai, China

Corresponding authors: E-mail address: [email protected] (Lei Zhu*) E-mail address: [email protected] (Zhen Huang*) * Corresponding author address: Key laboratory of Power Machinery and Engineering , Shanghai Jiao Tong University, Shanghai 200240, China. Tel: +86-21-34205949 Fax: +86-21-34205949

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Abstract Experimental and chemical kinetics studies were carried out to compare soot particle morphology evolution in atmospheric pressure laminar co-flow diffusion flames of methyl butanoate (MB) and ethyl butanoate (EB), in order to investigate the effects of alcohol chain in biodiesel molecular on soot formation. The thermophoretic sampling technique was used to capture soot particles directly at different heights along flame centerlines. Transmission electron microscopy (TEM) was applied to obtain particle morphology information. Moreover, total sampling followed by thermogravimetric analysis and Raman spectroscopy analysis was performed to study soot oxidation reactivity and degree of disorder in soot structure. The results show that the flame structures and temperature profiles along flame centerlines of two test esters are similar. The primary particle diameters of EB are larger at almost all the sampling positions and soot inception and aggregation process occur earlier in EB flame. The phenomenon can be illustrated through simulation works which indicated EB could be decomposed quickly at lower temperature because of the six-centered unimolecular elimination reaction, leading to earlier and stronger formation of C2H4, and then promoting the formation of C2H2. It is also found the oxidation reactivity of soot from MB flame becomes higher compared with EB. Keywords: Biodiesel surrogates; Methyl butanoate; Ethyl butanoate; Soot morphology; Oxidation reactivity; Alcohol chain

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1. Introduction

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The energy demand around the world is increasing sharply, with the continuous consumption of

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traditional fuels, especially the petroleum-based energy. Biodiesel is a potential alternative fuel to

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meet the energy need and alleviate the damage on our environment in the meantime1. Pure

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biodiesel is comprised of methyl or ethyl esters of long-chain fatty acids, which is produced from

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renewable sources such as vegetable oils or animals fats2, 3. Recently, the utilization of biodiesel in

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diesel engine has been widely studied, which indicates that the biodiesel can reduce the emissions

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of total particulate mass, carbon monoxide and unburned hydrocarbons compared to fossil diesel

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fuel in diesel engine, while increase particle number concentration 2, 4, 5.

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Soot particles are formed through complex physical and chemical processes6, 7, which can be

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influenced by many factors, such as fuel types and combustion condition. Suppression the

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formation of particulate matter emission of biodiesel in diesel engine requires detailed

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understanding about impact factors on the formation and evolution of soot particles. Recently,

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more attention has been paid to the effect of fuel molecular structure on soot formation during the

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combustion of biodiesel or their surrogates. Schönborn et al.8 introduced three distinct aspects of

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molecular structure of biodiesel (fatty acid chain length, degree of unsaturation and alcohol chain

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length). It was observed from their work that the molecular structure mentioned above did have a

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significantly affect on the formation of soot particles on a single-cylinder engine. Lei Zhu et al.9

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conducted experimental study on a 4-cylinder direct-injection diesel engine using diesel fuel

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doped with some fatty acid esters in order to get insight into the effects of typical chemical

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structure of esters on the combustion and emission characteristics. It was found that with the

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increase of fatty acid chain length, the nucleation number concentration decreased while 3


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accumulation increased. Compared with methyl ester, ethyl ester generated higher accumulation

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number concentration, with larger primary particle diameters. These results are meaningful for the

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optimization of biodiesel composition.

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Considering that the complex combustion process in engine can increase the analysis difficulty

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and result uncertainty, many studies of soot formation have been conducted in laboratorial flames.

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Kholghy and his coworkers10 conducted the experimental study in laminar co-flow diffusion

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flames of 5-decene, 1-decene, n-decane and a B100 biodiesel surrogate (a mixture of 50%

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n-decane/ 50% methyl octanoate) in order to explore the structural effects on soot formation. The

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thermophoretic sampling method with TEM was applied to capture particles directly from flames

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and compare soot morphology evolution. It was indicated that the presence of ester moiety could

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suppress soot formation while unsaturation markedly promoted soot inception and growth.

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Merchan et al.4 investigated the formation and evolution of soot particles in the aspect of

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morphology and nanostructure through TEM and HR-TEM in wick-generated laminar diffusion

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flames of different biodiesels and diesel fuel with the thermophoretic sampling method. It was

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concluded that the soot primary particles derived from three biodiesel flames had an obviously

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smaller mean diameters than those from diesel fuel at each sampling location along the flame

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centerline. Moreover, certain distinction in terms of the primary particle diameters also existed

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within the test biodiesel flames. The authors attributed these differences to the different centerline

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temperature profiles caused by distinguishing fuel characteristics and indicated that the higher the

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temperatures were, the higher the oxidation rate of the soot particles was, resulting in smaller

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primary particle diameters. Maricq11 studied the comparison of soot formation in methyl butanoate,

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soybean biodiesel and petroleum fuel in co-flow diffusion flames. Differential mobility analysis 4


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and laser ablation particle mass spectrometry were used to measure the size distributions and

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composition of soot particles respectively. It was found that methyl butanoate flame produced a

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lower soot level than those from the other flames. In addition, the soot formation process of

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methyl butanoate exhibited a delayed trend compared to soybean biodiesel and ultra-low sulfur

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diesel. Qiyao Feng and his coworkers12 compared sooting propensities of different typical esters in

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counter-flow flames. In order to better understand the experimental results, they used chemical

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kinetic mechanisms to describe the formation of C2H4, C2H2 and C3H3 and found that the sooting

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propensity were closely related to the concentrations of these three key species.

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In order to reduce the soot emission from diesel engines, diesel particulate filter (DPF) has been

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widely applied in diesel engine as an efficient after-treatment device. Regenerate process is

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required for DPF to oxidize the collected particles and maintain the filter backpressure. Therefore,

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the oxidation reactivity of soot particles produced by engines has important effect on DPF

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regeneration and need to be further studied. Barrientos et al.13 studied the effects of ester structure

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on soot oxidative reactivity with a co-flow diffusion flames of different methyl esters, real

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biodiesel and diesel. It was observed that shorter alkyl chains in methyl esters lead to higher

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reactivity and the number and the location of the C=C bonds have noteworthy effect on soot

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oxidation reactivity.

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Most of the fundamental studies focused on the effect of ester moiety, fatty acid chain length

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and degree of unsaturation on both soot formation and oxidation reactivity. As one of the

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important molecular structures, the effects of alcohol chain were just investigated in some studies

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using methyl and ethyl esters with the same carbon atoms number in molecules. Considering that

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actual biodiesels always contain fatty acid methyl esters or fatty acid ethyl esters with the same 5


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aliphatic main chains due to different production processes, it is important and necessary to

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investigate the effects of different alcohol chains with the same fatty acid chain length on the

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evolution of soot morphology and soot oxidative reactivity. Two kinds of pure esters, methyl

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butanoate and ethyl butanoate, were used in the present study because the only difference of MB

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and EB is one and two carbon atoms in their alcohol chain. The short fatty acid chain length of

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MB and EB can weaken the influence of main carbon chain and highlight the effects of alcohol

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chain on soot formation process and soot characterization.

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The overall goal of this study is to compare the soot characteristics of MB and EB to investigate

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the effects of different alcohol chains with the same fatty acid chain length on the soot formation

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and soot oxidative reactivity. Thermophoretic sampling method was applied to capture particles

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inside the flame and the soot morphology information was obtained from images of transmission

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electron microscopy (TEM). In addition, soot sampled at the flame tips through total sampling

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system was analyzed by TGA and Raman spectroscopy to obtain the information about the

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oxidative reactivity and microstructure respectively.

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2. Experimental methods

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2.1. Test fuels

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The components of biodiesel are extremely complex, making it difficult to clearly understand

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the effect of fuel composition and molecular structures on soot formation. As a result, it is

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reasonable to firstly investigate some pure, simple methyl/ethyl esters, such as methyl butanoate

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and ethyl butanoate. In this work, methyl butanoate and ethyl butanoate were used in the co-flow

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laminar diffusion flame experiments. Both fuel molecules contain 4 carbon atoms in main carbon

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chain and no double bond. 6


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2.2. Co-flow laminar diffusion flame Burner

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Fig. 1 shows the experimental apparatus in detail. An atmospheric pressure laminar co-flow

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diffusion flame burner was utilized in present work. The burner consists of three concentric brass

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tubes. The central tube supplied the fuels, which were previously vaporized and delivered to the

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burner using N2 as carrier gas with a flow rate of 238 mL/min. The intermediate tube was used to

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supply air as oxidizing gas with a flow rate of 14.8 L/min. The outer tube was used for N2 at 16.2

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L/min as a shield to protect flames from the interference of the surrounding air. The inner

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diameters of three tubes are 10, 27 and 59 mm respectively. The tube edges are sharpened to

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reduce shear at the boundaries between flows. In order to make flows uniform and improve the

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flame stability, a large amount of tiny glass beads were filled in the annulus between the air tube

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and the central fuel tube, forming a 2 cm thick layer. The burner was attached well to a

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three-dimensional coordinate frame that could be adjusted with a precision of 0.1 mm.

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Fig. 1 Schematic of the experimental setup

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Liquid fuel was delivered via a syringe pump (LSP01-1BH, Baoding Longer Precision Pump

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Co., Ltd. )with an accuracy of ± 0.5% over the flow range. The liquid fuel was aerosolized by a

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nebulizer using the carrier gas (N2). The exit of the nebulizer was inserted into a quartz chamber

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covered with heating elements. Considering that the boiling points of MB and EB are about

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102 °C and 121 °C respectively, the quartz chamber, the fuel transfer line to the burner and the

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burner were all heated with heating elements to prevent fuel condensation. The setting temperature

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of these heating elements was 190 °C, ensuring that the inner temperature was above 180 °C,

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which was well above the boiling points of the two tested fuels and could promote volatilization.

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In addition, the air and protecting gas were also preheated to 140 °C by heating tape before the

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entrance to the burner.

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A constant carbon atom flow rate for different fuels leads to a similar flame size and shape in

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co-flow diffusion flame, which can keep the residence time and velocity fields similar13. In the

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present study, the carbon atom flow rates for MB and EB were maintained at 0.25 g/min, with the

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liquid fuel flow rate of 28.36 mL/h and 27.59 mL/h respectively.

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2.3. Flame temperature

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The rapid thermocouple insertion method, which has been used extensively in the temperature 14-16

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measurement of soot laden flame

, was applied to measure the temperature profiles in the

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flame volume. To minimize the effect of soot buildup in the soot-containing region of the flame,

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the thermocouple was inserted rapidly into the desired position and the temperatures were quickly

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recorded while the emissivity and effective diameter were not affected considerably. Moreover,

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before the next insertion to the desired location, the thermocouple had to be cleaned near an

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oxidizing region of the flame to burn the deposited soot on the surface of its junction and wires.

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The configuration of temperature measurement apparatus is shown in Fig.2. In this work, the

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temperature measurements of the co-flow diffusion flames were performed with an uncoated S 8


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type thermocouple (Pt-Pt/13% Rh) with a wire diameter of 127 µm and a junction diameter of 240

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µm (Omega Engineering, Inc). The thermocouple was not coated because the catalytic effects

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were negligible due to the low radical concentration in diffusion flames15. The temperature data

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was acquired using a temperature & process controller (DP16Pt, Omega Engineering, Inc)

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connected with a computer that recorded the online temperature profiles through a matched

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software.

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Fig. 2 Schematic of the temperature measurement apparatus

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As shown in Fig. 3, in the soot-free location, the plateau temperature after the rapid increase

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during the thermocouple response time is corresponding to the temperature of the thermocouple

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junction (averaged value in the blue dotted line circle shown in Fig. 3). However, in a

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soot-containing region, two stages during the decrease of temperature can be identified. The first

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one is known as the “variable-emissivity stage” and the second one is the “variable-diameter

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stage”. In a soot-containing region, the maximum value is selected as the corresponding measured

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temperature (peak value in the red dotted line circle shown in Fig. 3) 16.

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Fig. 3 Temperature history of the thermocouple at soot free region and soot containing region

Radiation corrections were performed with the method suggested by Shaddix17:

.

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where is the gas temperature (K), is the thermocouple junction temperature (K), is

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the ambient temperature (293 K), is the emissivity of the thermocouple junction, is

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Stefan-Boltzmann constant (5.67E-08 W·m-2 K-2), is the diameter of the thermocouple junction

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(m), is the thermal conductivity of the gas (W·m-1 ·K-1) and is the Nusselt number.

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The emissivity of the uncoated thermocouple at different temperatures was acquired from18.

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Nusselt number was evaluated to be 2.26 for the 240 µm diameter junctions in co-flow diffusion

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flame using the low-Peclet number expansion for spheres of Acrivos and Taylor19. The thermal

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conductivity of the gas was estimated using the data from20. This radiation corrections method is

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widely used for the temperature profiles of the co-flow diffusion flame16, 21, 22. 10


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2.4. Soot sampling

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2.4.1. Thermophoretic Sampling

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Thermophoretic sampling (TS) technique was applied to capture particle samples from the

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visible flame for subsequent TEM analysis. The schematic of the sampling system is shown in

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Fig.1. The TEM grid was held by a TS probe, which was aligned to be parallel with the fuel and

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air flow. The tine of the TS probe was as thin as 0.3 mm in order to reduce perturbation to the

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flame. An electric cylinder (DNCE-32-320-LAS-H, FESTO) with a linear motor controlled by a

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motor controller (SFC-LACI-VD-10-E-H2-I0, FESTO) attached the TS probe and made it insert

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and leave out the flame in a short and controllable time. Soot particles were driven

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thermophoretically to the cold surface by the temperature gradient between the cold surface of the

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TEM grid and the hot flame gas23, 24. In the present work, the residence time of the grid was kept

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the same for all the sampling position, making it possible to qualitatively compare soot volume

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fraction along the centerline directly on the basis of the degree of soot coverage in TEM images.

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According to the results of preliminary experiments for thermophoretic sampling, if the residence

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time was shorter than 35 ms, there was less particles obtained in the grid at most sampling

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positions, not only making it difficult to observe adequate samples in TEM view but also

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increasing the contingency for the image analysis. If the residence time was longer than 45 ms ,

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too many soot samples were obtained on the grid in high soot-containing regions, leading to

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seriously widespread overlap of aggregates which increased the difficulty to observe the soot

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morphology and distinguish the primary particles. In addition, a too long resident time could make

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the ultrathin TEM grid damaged at high temperature regions in the flame. In the present work, the

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residence time of 42 ms was chosen. As the preliminary experiment results shown, with this 11


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residence time, enough samples were obtained for TEM observation and the soot coverage was

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maintained below 15% at all the sampling positions, which ensured the reliability of following

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analysis. Additionally, no damage of the grids was found even in the highest temperature position

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of the flame centerline. To minimize the unavoidable contamination when the grid and probe were

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passing through the flame towards the sampling position, the transit time was kept around 10ms.

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Both the exposure and transit time were set and controlled through computer software. Soot was

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sampled on the centerline of the co-flow diffusion flame at different heights. The variable height

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in this work represents the axial distance from the edge of the burner’s nozzle to the center of the

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TEM grid on the flame centerline.

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Sampling grids are carbon-supported 3.05 mm diameter TEM grids (300 mesh Carbon coated

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standard square copper girds). The soot samples were analyzed using a transmission electron

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microscope (Tecnai G2 spirit Biotwin) with an accelerating voltage of 120 KV. The magnification

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range was between 50 and 340,000 times. The TEM micrographs were collected on a Gatan digital

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imaging system and processed by digital micrograph software. The image processing software

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Image-Pro Plus 6.0 (Media Cybernetics) was applied to measure the primary particle diameters in

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TEM images with magnification of 98,000 times.

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2.4.2. Total sampling

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For the analysis of the soot characteristics (e.g. oxidation reactivity), soot particles at the

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post-flame region were sampled through a 10 cm length quartz probe with a 0.8 mm inner

232

diameter immediately above the flame tip. The probe with this diameter not only could guarantee

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enough soot samples on the filter for the following analysis, but also lessened the perturbation of

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the flame. In order to avoid the potential effect of soot deposition on the inner wall of the probe, it 12


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was single-used for one sampling process. The quartz probe was connected to a vacuum system in

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line with a quartz filter (0.2 µm pore size) for the collection of soot particles, which was fixed and

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protected by a filter holder. Between the quartz filter and the vacuum system, a cold trap was used

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to capture the volatile material. The sampling time was kept constant at 10 min to ensure enough

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material captured on the filter. Then, the samples obtained were well stored for TGA and Raman

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analysis. Similar sampling system has been used by Santamaría et al.25

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2.5. Thermogravimetric analysis (TGA)

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In this study, a thermogravimetric analyzer (TA discovery) with a weighing resolution of 0.001

243

mg and a temperature precision of 1 K and was employed to evaluate the oxidation activity of soot

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samples obtained from the co-flow diffusion flames of MB and EB. The particle samples were

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peeled off from the filter and then placed in a platinum pan. The samples were initially arranged in

246

a nitrogen atmosphere. After an isothermal process for 10 min at 40 °C for stabilizing the samples,

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the temperature was heated to 400 °C at a rate of 30 °C/min and maintained isothermally for 30

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min under nitrogen condition. After this procedure, the volatile organic fraction (VOF) of soot

249

particles was completely removed. Subsequently, N2 was replaced by ultra zero air (99.0% purity)

250

at a flow rate of 100 mL/min. Meanwhile, the temperature was ramped to 800 °C at a rate of

251

15 °C/min in order to accomplish oxidation of the samples. The sample mass loss during the

252

oxidation process of soot particles was normalized according to the weight after thermal treatment.

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2.6. Raman spectroscopy

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Raman spectra of the soot particle samples were obtained by a dispersive Raman microscope

255

system (SENTERRA R-200L) with a 532 nm He/Ne laser as an excitation source. The spectra of

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the samples were in the range of 80-4200 cm-1 with a spectral resolution of 3 cm-1. The soot 13


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samples on the quartz filters did not need pretreatment before the detection. For each sample,

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Raman spectra were collected at 5 different positions of the filter with an integration time of 10 s

259

to ensure the reproducibility and representativeness of the results.

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3. Results and discussion

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3.1. Temperature profiles

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Fig. 4 shows the temperature profiles on the centerline of the flames fueled with methyl

263

butanoate and ethyl butanoate respectively. It is observed that both temperature profiles exhibit a

264

rising trend as the height above the burner increases. Moreover, the temperatures of the MB and

265

EB flames are extremely close to each other at each sampling position, which indicate that there is

266

no significant effect of the molecular structure of the two test fuels on the temperature profiles.

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The results are consistent with those in the work by Schwartz et al. 26, in which they find that the

268

temperature profiles are almost identical in different co-flow diffusion flames, and demonstrate

269

that the overall flame structure is not affected by the ester types doped in the flame. In present

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work, the almost identical temperature profiles illustrate that the differences in the evolution

271

processes and particle characteristics are mainly due to different molecular structure of the two

272

test fuels.

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Fig. 4 Centerline temperatures at different axial locations of MB and EB flames

3.2. TEM analysis

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Considering the similar flame height and the almost identical temperature profiles, six regions

277

were defined in order to compare the soot formation and evolution in the flames of the two test

278

fuels in a consistent standard. A summary of the data about the six regions is presented in Table 1.

279

As can be observed in the table, the relative heights for the same regions of MB and EB flames are

280

reasonably close to each other. Furthermore, the temperatures in each region are fairly close, with

281

a maximum difference of 3.1% in Region 1.

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Table 1. Data summary about six regions in two test flames Region Fuel

1 MB

2 EB

MB

3 EB

4

MB

EB

5

MB

EB

MB

6 EB

MB

EB

Sampling 10

20

30

40

50

60

Height(mm) Relative Heighta(%)

14.1

13.2

28.2

26.5

42.4

39.7

56.5

52.9

70.6

66.1

84.7

79.4

Temperature(K)

931.4

902.8

1191

1156.6

1286.4

1306.6

1406.6

1425.8

1458.59

1491.5

1775.4

1769.4

Temperature 3.1

2.9

-1.6

-1.4

deviation(%)

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a: Relative Height=Sampling Height/Flame Height 15


-2.3

0.3

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3.2.1. Soot evolution on the centerline of the MB and EB flames

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Fig. 5 and Fig. 6 show a series of TEM micrographs of particles sampled thermophoretically at

286

different flame regions along the centerline of the laminar co-flow diffusion flames of methyl

287

butanoate and ethyl butanoate respectively. From the TEM images with 49,000 magnification in

288

Fig. 5, it can be observed that the degree of agglomeration and the morphology of the particles are

289

strongly influenced by the position within the flame. Furthermore, the 340,000 magnification

290

TEM images in Fig. 6 clearly reveal a strong variation of soot particle diameter along the

291

centerline of two test flames. The mean diameters of primary particles at different axial locations

292

of the MB and EB air-flames are shown in Fig. 7. In soot-containing regions ( height=40, 50 mm)

293

along the flame centerline, averaged primary particle diameters were determined by measuring the

294

diameters of hundreds of particles from several micrographs collected on the sampling positions.

295

In the soot-free regions ( height=10, 20, 30 mm) the number of measured particles was slightly

296

less due to the lack of soot and smaller thermophoretic force. In addition, near the flame tip

297

( height=60 mm ) where the oxidation was really drastic, the particles obtained and measured were

298

also less than those in the soot-containing regions. This measurement was repeatedly performed

299

four times for each picture. The standard deviations of reduplicative measurements were shown as

300

vertical uncertainty bars in Fig. 7.

16


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Energy & Fuels

301 302

Fig. 5 49K resolution TEM images of soot sampled along the centerline of two test flames

303 17


Energy & Fuels

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304 305 306

Fig. 6 340K resolution TEM images of soot sampled along the centerline of two test flames. The views in red dashed box are enlarged for clear observation of two typical particles with and without liquid-like appearances. The orange lines in enlarged images represent the distinguishable boundaries of particles.

307

From Fig. 5a and b, it can be observed that in Region 1 and 2 of the MB flame, only

308

polydisperse singlet particles exist with irregular outlines, which illustrates that in these regions,

309

an intense particle inception happens while little collision of particles occurs during this flame

310

conditions. The blue plot in Fig. 7 shows that the singlet particles become larger as the height

311

increase from Region 1 to Region 2 in an MB flame, as its flame temperature varies from 931 K to

312

1191 K. In an EB flame, small aggregates with liquid-like appearances, which are surrounded by

313

several small singlet particles, are found in Fig. 6g in Region 1. This phenomenon is different

314

from that observed in the same region of the MB flame. In the EB flame, the coexistence of single

315

particles and aggregates in the Region 1 indicates that both the particle inception and aggregation

316

occur in this zone, which is near the burner's nozzle. As we move along the centerline to Region 2

317

in the EB flame, the degree of agglomeration increases observably, with more and larger

318

aggregates compared to the Region 1. It is illustrated from the enlarged images (Fig.6m) that the

319

aggregates still shows obvious liquid-like appearances. Inside the blurry boundaries of the total

320

aggregates (orange lines in Fig.6m), the primary particle outlines cannot be distinguished.

321

Meanwhile, the primary particle size undergoes a drastic growth, with the temperature increasing

322

from 903 K to 1157 K.

18


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Energy & Fuels

323 324

Fig. 7 Mean diameters of primary particles measured at different axial locations of the two test flames

325

As the sampling position moves further away from the edge of the burner nozzle, aggregates of

326

particles firstly appear in Region 3 on the centerline of the MB flame, which can be seen from Fig.

327

5c. It is observed that the size of the aggregates increases while the mean diameter of primary

328

particles increases sharply from 23.6 nm to 30.4 nm, reaching the peak value among all sampling

329

locations of the MB flame. The size growth of primary particles and aggregates are mainly due to

330

surface reactions (including PAH condensation and hydrogen abstraction-C2H2 addition

331

mechanism) and coalescence23. In Regions 3 and 4 of the MB flame, the coexistence of aggregates

332

and single spherical particles leads to a bi-modal distribution of particle size, which has been

333

measured in other liquid fuel flames27. Moreover, it is observed from Fig. 6c and d that primary

334

particles in the aggregates possess blurry boundaries and still show some liquid-like appearance in

335

Region 3 and 4 of the MB flame. This phenomenon can be evidence that aggregation occurs

336

before the carbonization of primary particles is finished. As to the particles derived from the EB 19


Energy & Fuels

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337

flame, the mean diameter of primary particles reaches the peak value (38.5 nm) in Region 3

338

among all sampling positions on the centerline, and then undergoes a slight decrease to 36.5 nm

339

with the temperature increasing from 1307 K to 1426 K.

340

It is found from Fig. 5 and 6 that, in Regions 4, 5 and 6 of both MB and EB flames, in addition

341

to the soot particle aggregates, there appears plenty of small single particles with extraordinary

342

regular spherical shapes and considerably clear boundaries, which is quite different from the

343

morphology characteristics of soot particles. Based on further detection by the mapping technique,

344

it is proved that these single small regular particles are composed of copper. It can be inferred that

345

the copper covering the surfaces of grids melts when the temperatures of grids exceed the copper's

346

melting point (1358 K) within the flame. Then the copper condenses after the grids are removed

347

from the flame and cooled down to the ambient temperature. As mentioned above, the capture of

348

soot particles on the grid surfaces indicates the existence of sufficient temperature gradient

349

between grids and flame at the sampling position. Considering that the melting point of copper is

350

fairly close to the flame temperatures of MB and EB in Regions 4, 5 and 6, it would be extremely

351

impractical to produce an adequate temperature gradient if the grid temperature is around the

352

melting point of copper. Therefore, the melting process of copper on the grids can only happen

353

after the soot particles are captured on the grids. As a result, the copper particles have minimal

354

influence on the evolution of soot particles.

355

It can be observed from Fig. 5e and k that few single soot particles exist and almost all the

356

primary particles are in large clusters in Region 5 of the MB and EB flames. From Fig. 6e and k, it

357

can be seen that the primary soot particles in this region become mature with clear boundaries and

358

more uniform sizes, compared with the particles in lower regions, which indicates the formation of 20


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Energy & Fuels

359

fully carbonized soot aggregates, as Kholghy suggested23. The enlarged image (Fig.6n) typically

360

shows this mature morphology of soot particles before strong oxidation. Then the soot volume

361

fraction and degree of particle agglomeration significantly decrease as the gas flow moves towards

362

higher region near the tip of the MB and EB flames. Meanwhile, the mean diameter of the primary

363

particles in Region 6 of the two test flames reduces dramatically below 15 nm. The sharp decrease

364

of mean diameter in Regions 5 and 6 is the result of intense oxidation reactions in a considerably

365

high temperature range28, 29. It is observed from Fig. 4 that the temperature plots on the centerline

366

of both MB and EB flames experiences a lower increasing rate in Regions 4 and 5. Based on the

367

work in23, more carbonized soot particles have a higher emissivity than immature particles at

368

lower flame regions, which increases radiated heat from the particles, resulting in the decreasing

369

slope of the temperature plots. Subsequently, the temperatures of MB and EB flames rise sharply

370

after Region 5 because the strong oxidation in Region 6 obviously reduced soot volume fraction

371

and soot aggregate degree, leading to a decrease of radiant heat loss from the flame and

372

furthermore, the strong particle oxidation promote the release of the energy embedded in soot.

373

3.2.2. Comparison of soot from MB and EB flames

374

From the TEM images in Fig. 5 and Fig. 6, it is noteworthy that in the EB flame, aggregates are

375

detected in Region 1 near the burner's nozzle, while those are first found in Region 3 in the MB

376

flame. Furthermore, in lower regions of the flame(Region 1, 2 and 3), the degree of particle

377

agglomeration for EB is much higher than those of the MB flame. From the phenomenon

378

mentioned above, it can be inferred that soot formation process in EB flame are shown to occur

379

earlier with a stronger rate compared with MB flame.

380

From the profiles of primary particle diameter as shown in Fig. 7, it is observed that the primary 21


Energy & Fuels

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381

particle diameters of EB flame are larger than those of MB flame at almost all the sampling

382

positions except Region 1. In Region 1 (10 mm), the number of soot particle samples in TEM

383

images is not large enough to ensure credible statistical results. Moreover, the particles in this

384

region show liquid-like appearances with blurry outlines, making it difficult to measure their

385

diameters. Therefore, the data in Region 1 cannot fully represent the actual results. There are two

386

explanations for the distinction in primary particle diameters. First, methyl butanoate has an

387

oxygen content of 31.3% by mass, higher than that of ethyl butanoate which is 27.5%. The higher

388

oxygen content is expected to enhance its capacity to oxidize soot particles, resulting in a

389

suppression of soot size growth. Second, the thermal decomposition of the EB in the flame

390

condition may lead to a more intense formation of intermediate species which promote the soot

391

mass surface growth (e.g., C2H2) than that of the MB.

392

Acetylene (C2H2) is generally proved to be key species which contributes to the formation of

393

larger PAHs, soot inception and surface growth via the hydrogen-abstraction-C2H2-addition

394

(HACA) mechanism. Ethylene (C2H4) is the important intermediate product correlating with the

395

formation of the C2H2. The formation of C2H4 and C2H2 can be described more credibly by current

396

chemical kinetic models of methyl and ethyl esters than that of the larger species during the

397

subsequent soot-forming reactions12. As a result, in order to get insight into the earlier soot

398

formation in EB flame and the differences of the primary particle diameters of both MB and EB,

399

detailed chemical kinetic mechanisms of methyl butanoate and ethyl butanoate30 were used to

400

focus on the concentration variation of C2H4 and C2H2. Aiming to compare the decomposition

401

process of MB and EB over a range of temperatures, simulation work was performed with the

402

Plug Flow Reactor (PFR) module of the CHEMKIN Pro package. In this work, the parameters 22


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Energy & Fuels

403

were set to model a flow reactor with a heated section length of 550 mm and an inner diameter of

404

8 mm, operating at atmospheric pressure, under a diluted condition by nitrogen gas and over a

405

temperature range 700–1400 K. The mole fractions of the inlet gas for MB and EB were 2% and

406

the flow rates of inlet gases were maintained at 0.015 mol/s, which can guarantee a laminar flow

407

condition. The equivalence ratios of 100 was selected to approach the anoxic condition in region 1

408

to 3 where the fuel decomposition occurs. The detailed initial setup conditions are presented in

409

Table 2.

410

Table 2. Initial setup conditions for the simulation with Plug Flow Reactor (PFR) module Composition of inlet gases

Fuel+ oxygen+ nitrogen

Fuel Type

MB or EB

Inlet gases flow rates (mol/s)

0.015

Fuel mole fractions in inlet gases

2%

Equivalence ratios

100

Heated section length (mm)

550

Reactor Inner diameter (mm)

8

Pressure (atm)

1

temperature range (K)

700-1400

411

The variations of the species mole fractions of the two test fuels, ethylene and acetylene at the

412

end of the heated section are shown as the function of the setting temperature in figure 8. As it can

413

be observed in Fig. 8 that the decomposition of EB occurs earlier at around 800 K with a complete

414

consumption beyond 1100 K while the concentration of MB starts to decrease at around 950 K

415

and maintain zero from 1300 K to higher temperature. It can be illustrated that EB tends to

416

decompose at a lower temperature compared with MB and has an evidently higher reactivity under

417

the investigated conditions. As the decomposition of MB and EB occurs, the concentration of

418

C2H4 derived from two test fuels starts to increase respectively, which can be shown as dash lines

419

in Fig.8. It is noteworthy that the formation of C2H4 starts earlier and the production of C2H4

420

derived from EB is obviously higher than that from MB over a wide temperature range from 800 23


Energy & Fuels

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421

K to 1400 K. According to Fig. 8, the concentrations of C2H2 from both fuels start to increase

422

from around 1150 K and the production of C2H2 from EB is notably higher than that of MB, which

423

can promote the surface growth and lead to larger primary particle diameters.

424 425 426

Fig. 8 Computed mole fractions of fuels (MB, EB) and their decomposition products (C2H4, C2H2) at the end of the heated section as a function of modeling temperature

427 24


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Energy & Fuels

428 429

Fig. 9 Computed mole fractions of fuels (MB, EB) and their decomposition product (C2H4) along the axial distance

430

The species mole fractions along the axial distance are shown in Fig. 9 at temperature of 1100

431

K for MB and EB. It can be illustrated that EB was completely consumed immediately it went into

432

the reaction zone while MB decomposed relatively slowly. As a result of the rapid decomposition

433

of the fuel, the concentration of C2H4 derived from EB increased sharply and maintained at a

434

higher level compared with MB. Rate of production analysis was performed to determine the

435

reaction pathways, which are related to the formation of C2H4. The setting temperatures of 1100 K

436

and the axial distance of 6 cm was selected for the analysis.

at modeling temperature of 1100 K

437 438 439 440

Fig. 10 Reaction pathways for C2H4 formation from MB (a) and EB (b) at modeling temperature of 1100 K and the

441

Fig. 10 display the major pathways leading to the formation of C2H4 for two test fuels. It can be

442

seen from Fig. 10(a) that the C2H4 is produced through four main pathways from MB. Pathway

443

One is that MB undergoes H-abstractions mostly via ·CH3 and ·H to form two isomer radicals

444

(R1-R4).

axial distance of 6 cm. Each number indicates the percentage contribution to the formation of the species that is pointed by the arrow.

445

MB + ·CH3 → CH3CH2·CHC(O)OCH3 + CH4 (R1)

446

MB + ·H → CH3CH2·CHC(O)OCH3 + H2 (R2)

447

MB + ·CH3 → CH3CH2CH2C(O)OCH2· + CH4 (R3) 25


Energy & Fuels

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Page 26 of 46

MB + ·H → CH3CH2CH2C(O)OCH2· + H2

448

(R4)

449

Then these two isomer radicals decomposes through several steps of reactions to form the radical

450

of ·CH2CH2CH3, which contributes to the largest part (38.3%) of total C2H4 formation via the

451

following reaction. ·CH2CH2CH3 → C2H4 + ·CH3

452

(R5)

453

Pathway Two is that MB decomposes through H-abstractions to produce ·CH2CH2CH2C(O)OCH3

454

radical. Then this radical decomposes to form 35.9% of C2H4 production via the reaction as

455

follows.

456

·CH2CH2CH2C(O)OCH3 → C2H4 + ·CH2C(O)OCH3 (R6)

457

Pathway Three is that MB undergoes unimolecular reaction to generate ·CH2CH3 radical. Then

458

this radical reacts with the third body (+M) to form 18.0% of the C2H4 through the following

459

reaction.

460

·CH2CH3 + M → C2H4 + ·H +M (R7)

461

Pathway Four is that the decomposition of MB leads to the formation of ·CH2CH2C(O)OCH3,

462

which then decomposes to produce the smallest part (7.8%) of C2H4 formation through the

463

following reaction. ·CH2CH2C(O)OCH3 → C2H4 + ·C(O)OCH3

464

(R8)

465

The pathway of the formation of C2H4 from EB is obviously different from that of MB, which

466

can be seen from Fig. 10(b). Most of the C2H4 (91.9%) are produced directly from EB through an

467

unimolecular dissociation reaction (R9), which benefits from a transition state of a six-membered

468

ring (Fig. 11)30

469

EB → C2H4 + CH3CH2CH2C(O)OH (R9) 26


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Energy & Fuels

470

Compared with MB, EB can decompose to form butanoic acid and C2H4 straightway, which is

471

the dominating pathway for the consumption of EB. On the basis of the work performed by

472

Schwartz and his coworkers26, it can be demonstrated that the higher decomposition rate of EB is

473

related to the six-centered unimolecular dissociation reaction, which MB hardly undergoes

474

because its alcohol chain is too short to establish an intermediate six-centered ring with carbonyl

475

group and the single-bonded oxygen atom. The earlier and stronger formation of C2H4 from EB

476

(shown in Fig. 8) is mainly due to the fast decomposition via the six-centered unimolecular

477

elimination, directly forming almost all of the C2H4.

478 479

Fig. 11 Six-centered unimolecular dissociation reaction for ethyl butanoate

30

480

Aiming to understand the formation process of C2H2, reaction path analysis was carried out to

481

identify the main pathways leading to C2H2 formation at the modeling temperature of 1300 K for

482

the end position of the heated section. For both MB and EB, the results show that more than 94%

483

of C2H2 was produced from ·C2H3 radical. The C2H4 contributes to about 64% of the formation

484

of ·C2H3 via the following pathways.

485

C2H4 + ·H → ·C2H3 + H2

(R10)

486

C2H4 + ·CH3 → ·C2H3 + CH4 (R11)

487

C2H4 + ·OH → ·C2H3 + H2O (R12)

488

It can be indicated that C2H4 plays a crucial role in the C2H2 formation, explaining the higher

489

yield of C2H2 from EB, which is consistent with the higher concentration of C2H4 under the 27


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

490

investigated modeling condition.

491

According to the above discussion based on simulation work, EB has a higher reactivity and can

492

decompose quickly at a lower temperature leading to an earlier and stronger formation of C2H4

493

due to the unimolecular dissociation reaction, compared with MB. As temperature increases, a

494

larger amount of C2H2 can be produced as a result of the higher concentration of C2H4 mainly

495

generated from EB decomposition. Considering the promoting effect of C2H2 on larger PAHs

496

formation and the surface mass growth, the above simulation results can provide an explanation

497

about the earlier soot formation process in EB flame.

498

3.3. Soot oxidative reactivity analysis

499

Soot samples used for TGA and Raman analysis were obtained immediately above the tips of

500

the two test flames by total sampling system. Fig.12 shows the results of thermogravimetric

501

analysis after removal of the volatile organic fraction (VOF) for soot samples generated from the

502

co-flow diffusion flames of methyl butanoate and ethyl butanoate respectively. The maximum

503

mass loss rate temperature (MMLRT) is defined as the temperature at which the mass loss rate

504

reaches its maximum value31. This temperature is also known as maximum rate of soot oxidation

505

temperature indicating how active the soot is in oxidation. The lower the MMLRT is, the more

506

active the carbon soot will be in oxidation32.

28


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Energy & Fuels

507 508

Fig. 12 Thermogravimetric analysis of MB and EB soot

509

From the normalized mass loss profiles in Fig.12, it can be observed that the MB soot

510

experiences more rapid mass loss than the EB soot before their percentages of initial mass

511

decrease to approximately 20%. Furthermore, in the mass loss rate profiles of Fig.12, it can be

512

seen that the MMLRT of methyl butanoate (488 K) is significantly lower than that of ethyl

513

butanoate(546 K), which suggests that the soot obtained from methyl butanoate diffusion flame

514

exhibits a higher oxidation reactivity than the ethyl butanoate soot. It is indicated that the structure

515

of alcohol chain in fatty acid esters has an impact on the soot reactivity. Specifically, the oxidation

516

activity becomes lower with the increase of alcohol chain length in these two fuels. This

517

phenomenon can be explained from three aspects. Firstly, the MB and EB undergo quite different

518

processes of decomposition and product formation, which may exert influence on the

519

nanostructure of soot particles. Hereinafter the Raman spectra results are good evidence for this

520

effect. Compared with the soot from the EB flame, that derived from the MB flame has a higher

521

oxidation activity as it possesses a more disordered carbon inner-structure. Secondly, as shown in 29


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

522

Fig.13, the soot particles derived from the MB flame have a higher VOF content than that from

523

the EB flame. Yehliu33 speculated that the internal surface area of soot particles increases with a

524

fast initial rise for micro-pores, due to the soot devolatilization. A higher VOF content leads to a

525

larger internal surface area during the devolatilization process and as a consequence, the

526

penetration and reaction of oxygen is promoted and the soot reactivity is increased. Thirdly, the

527

mass fraction of oxygen in MB (31.3%) is slightly higher than that in EB (27.5%). Compared with

528

EB, the higher fuel embedded oxygen content in MB may lead to more oxygen groups on the

529

surfaces of MB soot, which can be one of the reason for higher VOF content in MB soot. The

530

higher oxygen content in MB soot not only provides more active sites through the devolatilization

531

process but also can modify the soot microstructure to "capsule type oxidation"34 during

532

thermogravimetric analysis after removal of VOF , which can result in more quick oxidation

533

compared with EB soot.

534 535 536 537

Fig. 13 Volatile organic fraction of MB and EB soot

3.4. Raman spectroscopy analysis As a result of the sensitivity to structural differences of carbon-based materials, Raman 30


Page 30 of 46

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Energy & Fuels

538

spectroscopy has been used as a good diagnostic technology to investigate the microstructure of

539

various carbonaceous materials13. As shown in Fig.14, the black solid lines represent the

540

first-order Raman spectra in the range from 800 to 2000 cm-1 of soot particles sampled from the

541

MB and EB flames, typically exhibiting two overlapping peaks( the G peak and D peak). The G

542

peak rises due to the natural sp2 vibration mode of the six-member ring planes in the graphite-like

543

structure, and as a result represents the characteristics of an ideal graphitic structure. The D peak is

544

related to the nature of disordered graphite, which is attributed to manifestation of the in-plane

545

vibrational mode at the surface of the sp2domains35. With the first-order Raman bands, the degree

546

of disorder in carbonaceous materials can be evaluated by the integrated intensity of the D peak

547

relative to that of the G peak (i.e., Id/Ig). A higher ratio indicate a less graphitic structure of

548

carbon-based materials(e.g. soot particles).

549

Based on the method proposed by Jawhari et al.36, a three band curve fitting method was

550

employed to get more accurate spectroscopic parameters and properly determine the Id/Ig ratios.

551

The peak fitting are performed and shown as the dash lines in Fig.14. The two Lorentzian bands at

552

around 1338 and 1597 cm-1 corresponds to D band and G band respectively. The overlap between

553

the two peaks in the first-order Raman spectra is due to a D' band which is fitted as a Gaussian

554

shape appearing at about 1560 cm-1 and attributed to amorphous carbon fractions of soot.

555

As shown in Fig.14, the Id/Ig ratios for the soot particles derived from the tip of the methyl

556

butanoate flame is 2.78, which is relatively higher than that of the ethyl butanoate flame (2.51). It

557

is illustrated that the soot particles sampled at the post-flame region of MB have more disordered

558

and amorphous structures than those obtained at the same region of EB. More amorphous and

559

disordered soot is more reactive to oxidation37, 38. Therefore, the results obtained from the Raman 31


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

560

analysis are in good accordance with the observations from the TGA analysis, indicating that the

561

soot derived from MB diffusion flame has higher oxidation reactivity than the EB soot.

562 563 564 565

Fig. 14 Raman spectra analysis of MB and EB soot ( The intensity is normalized by the peak value of G band respectively)

4. Conclusions

566

A comparison of the soot morphology evolution along the centerline was conducted in a

567

pressure laminar co-flow diffusion flame of methyl butanoate and ethyl butanoate. In addition, the

568

oxidation reactivity of the soot particles sampled immediately above the tips of the two test flames

569

was investigated. The conclusions are drawn as follows:

570

(1)There is no significant impact of the molecular structure of the two test fuels on the temperature

571

profiles along the centerline of the co-flow diffusion flames, as both temperature profiles exhibit

572

similar trends and have almost identical values.

573

(2) Soot particles in the MB and EB flames exhibit a similar evolution trend. In the lower flame

574

region, an intense particle inception occurs. With increasing height, the degree of agglomeration 32


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Energy & Fuels

575

increases and the primary particle diameter grows as a result of surface growth and collisional

576

aggregation. Closer to the tip of the flame, both the primary particle diameter and aggregate size

577

reduce sharply due to the oxygen entrainment and high temperatures.

578

(3) The primary particle diameters of the EB flame are larger than those of MB flame at almost all

579

the sampling positions except Region 1 and the inception and aggregation process are inferred to

580

occur earlier and with a stronger rate in the EB flame than the MB flame. The phenomenon can be

581

explained by simulation results that EB can be decomposed quickly at a lower temperature

582

because of the six-centered unimolecular elimination reaction, promoting an earlier and stronger

583

formation of C2H4, and then producing a larger amount of C2H2, compared with MB.

584

(4) Through TGA analysis, the oxidation activity becomes lower with the increase of alcohol chain

585

length in these two esters, which are consistent with the Raman spectra results indicating that soot

586

particles from the MB flame have more disordered and amorphous microstructures than those

587

from the EB flame.

588

To sum up, as a typical characteristic of the molecular structure of biodiesels, the structures of

589

alcohol chains in MB and EB do have an effect on soot formation and evolution in aspects of

590

primary particle diameter and soot inception and aggregation process. Additionally, compared

591

with EB, methyl group in MB can lead to higher soot oxidation activity, which is beneficial for

592

reducing the soot emission in after-treatment devices of diesel engines.

593

Acknowledgements

594 595

The authors would like to thank the National Science Foundation of China (Project No. 51436005; 51676125) and the Foundation of Shanghai Jiao Tong University.

596 33


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663

Table 1. Data summary about six regions in two test flames Region Fuel

1 MB

2 EB

MB

3 EB

4

MB

EB

5

MB

EB

MB

6 EB

MB

EB

Sampling 10

20

30

40

50

60

Height(mm) Relative Heighta(%)

14.1

13.2

28.2

26.5

42.4

39.7

56.5

52.9

70.6

66.1

84.7

79.4

Temperature(K)

931.4

902.8

1191

1156.6

1286.4

1306.6

1406.6

1425.8

1458.59

1491.5

1775.4

1769.4

Temperature 3.1

2.9

-1.6

-1.4

-2.3

deviation(%)

664

a: Relative Height=Sampling Height/Flame Height

665 666

Table 2. Initial setup conditions for the simulation with Plug Flow Reactor (PFR) module Composition of inlet gases

Fuel+ oxygen+ nitrogen

Fuel Type

MB or EB

Inlet gases flow rates (mol/s)

0.015

Fuel mole fractions in inlet gases

2%

Equivalence ratios

100

Heated section length (mm)

550

Reactor Inner diameter (mm)

8

Pressure (atm)

1

temperature range (K)

700-1400

667 668 669 670 671 672 673 674 675 676 37


0.3

Energy & Fuels

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677

Figure Captions:

678


679


680


681


682


683

Fig. 6 340K resolution TEM images of soot sampled along the centerline of two test flames. The

684

views in red dashed box are enlarged for clear observation of two typical particles with and

685

without liquid-like appearances. The orange lines in enlarged images represent the distinguishable

686

boundaries of particles.

687

Fig. 7 Mean diameters of primary particles measured at different axial locations of the two test

688

flames

689

Fig. 8 Computed mole fractions of fuels (MB, EB) and their decomposition products (C2H4,

690

C2H2) at the end of the heated section as a function of modeling temperature

691

Fig. 9 Computed mole fractions of fuels (MB, EB) and their decomposition product (C2H4) along

692

the axial distance at modeling temperature of 1100K

693

Fig. 10 Reaction pathways for C2H4 formation from MB (a) and EB (b) at modeling temperature

694

of 1100K and the axial distance of 6cm. Each number indicates the percentage contribution to the

695

formation of the species that is pointed by the arrow.

696

Fig. 11 Six-centered unimolecular dissociation reaction for ethyl butanoate

697


698

Fig. 13 Volatile organic fraction of MB and EB soot 38


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Energy & Fuels

699

Fig. 14 Raman spectra analysis of MB and EB soot ( The intensity is normalized by the peak value

700

of G band respectively)

701 702 703 704

705 706


707

708 709


710

39


Energy & Fuels

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711 712


713

714 715


40


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Energy & Fuels

716 717


718

41


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

719 720 721 722

Fig. 6 340K resolution TEM images of soot sampled along the centerline of two test flames. The views in red dashed box are enlarged for clear observation of two typical particles with and without liquid-like appearances. The orange lines in enlarged images represent the distinguishable boundaries of particles.

723 724 725

42


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Energy & Fuels

726 727

Fig. 7 Mean diameters of primary particles measured at different axial locations of the two test flames

728 729 730

Fig. 8 Computed mole fractions of fuels (MB, EB) and their decomposition products (C2H4, C2H2) at the end of the heated section as a function of modeling temperature

43


Energy & Fuels

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731 732 733

Fig. 9 Computed mole fractions of fuels (MB, EB) and their decomposition product (C2H4) along the axial distance at modeling temperature of 1100K

734 735 736 737 738

Fig. 10 Reaction pathways for C2H4 formation from MB (a) and EB (b) at modeling temperature of 1100K and the axial distance of 6cm. Each number indicates the percentage contribution to the formation of the species that is pointed by the arrow.

739 740

Fig. 11 Six-centered unimolecular dissociation reaction for ethyl butanoate 44


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741 742


743 744

Fig. 13 Volatile organic fraction of MB and EB soot

45


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

745 746 747

Fig. 14 Raman spectra analysis of MB and EB soot ( The intensity is normalized by the peak value of G band respectively)

748 749

46


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Comparison of Soot Formation, Evolution, and Oxidation Reactivity of

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